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Armed men patrol around an infantry base in Perevalne, a village on the Crimean peninsula of Ukraine, on Sunday, March 2. Hundreds of armed men in trucks and armored vehicles surrounded the Ukrainian base Sunday in Crimea, blocking its soldiers from leaving.

Soldiers walk outside a Ukrainian military base in Perevalne as a local resident waves a Russian flag near them on March 2.

A Maidan self-defense unit member stands in support of Ukraine in Independence Square in Kiev, Ukraine, on March 2.

Demonstrators shout slogans during a rally in Independence Square on March 2.

Ukrainian soldiers, left, and unidentified gunmen, right, stand at the gate of an infantry base in Perevalnoye, Ukraine, on March 2.

Ukrainian soldiers guard a gate of an infantry base in Perevalnoye on March 2.

People attend a rally against Russia in Independence Square on March 2.

A young Ukrainian soldier stands behind the gate inside a Ukrainian military base that was surrounded by several hundred Russian-speaking soldiers March 2 in Crimea.

A soldier who was among several hundred that took up positions near a Ukrainian military base in Crimea stands near the base’s periphery on March 2.

An Orthodox clergyman stands at the gate of a Ukrainian military base in Crimea that was surrounded by several hundred Russian-speaking soldiers March 2. Behind him are Ukrainian soldiers standing just inside the gate.

A woman sheds tears during a rally in Independence Square on March 2.

Protesters hold flags of the United States, Germany and Italy during a rally in Independence Square on March 2.

People attend a morning prayer service at Independence Square on March 2.

Heavily armed troops, displaying no identifying insignia and who were mingling with local pro-Russian militants, stand guard outside a local government building in Simferopol, Ukraine, on March 2.

Protesters opposing ousted President Viktor Yanukovych sing Ukraine’s national anthem in Independence Square on Saturday, March 1.

A woman waits in front of unidentified men in military fatigues who were blocking a base of the Ukrainian frontier guard unit in Balaklava, Ukraine, on March 1.

U.S. President Barack Obama, in the Oval Office of the White House, talks on the phone March 1 with Russian President Vladimir Putin.

Troops stand guard in Balaklava on March 1.

Heavily armed soldiers displaying no identifying insignia maintain watch in Simferopol, Ukraine, on March 1.

People gather around the coffin of a man who was killed during clashes with riot police in Independence Square.

Armed men in military uniform block a Ukrainian military base in Balaklava on March 1.

Pro-Russian activists hold Russian flags during a rally in the center of Donetsk, Ukraine, on March 1.

Pro-Russian activists clash with Maidan supporters as they storm the regional government building in Kharkiv, Ukraine, on March 1.

A protester stands at a memorial March 1 for the people killed in clashes at Independence Square.

Armed men patrol outside the Simferopol International Airport in Ukraine’s Crimea region on Friday, February 28. Simferopol is the regional capital.

An image provided to CNN by a local resident shows Russian tanks on the move in Sevastopol, Ukraine.

Russian troops block a road February 28 toward the military airport in Sevastopol, Ukraine. The Russian Black Sea Fleet is based at the port city of Sevastopol.

Armed men stand guard in front of a building near the Simferopol airport on February 28.

An armed man wearing no identifying insignia patrols outside Simferopol International Airport on February 28.

Police stand guard outside the Crimea regional parliament building Thursday, February 27, in Simferopol. Armed men seized the regional government administration building and parliament in Crimea.

Police intervene as Russian supporters gather in front of the parliament building in Simferopol on February 27.

A man adds fuel to a fire at a barricade in Independence Square on February 27. Dozens of people were killed last week during clashes between security forces and protesters.

Pro-Russia demonstrators wave Russian and Crimean flags in front of a local government building in Simferopol on February 27.

Barricades in front of a government building in Simferopol on February 27 hold a banner that reads: “Crimea Russia.” There’s a broad divide between those who support the pro-Western developments in Kiev and those who back Russia’s continued influence in Crimea and across Ukraine.

Protesters stand in front of a government building in Simferopol on February 27. Tensions have simmered in the Crimea region since the Ukrainian president’s ouster.

Protesters in support of the president’s ouster rally in Independence Square, which has been the center of opposition, on Wednesday, February 26.

Security forces stand guard during clashes between opposing sides in front of Crimea’s parliament building in Simferopol on February 26.

Pro-Russian demonstrators, right, clash with anti-Russian protesters in front of a government building in Simferopol on February 26.

A police officer gets pulled into a crowd of Crimean Tatars in Simferopol on February 26. The Tatars, an ethnic minority group deported during the Stalin era, is rallying in support of Ukraine’s interim government.

A man places flowers at a barricade near Independence Square on February 26.

On February 26 in Kiev, A woman holds a photograph of a protester killed during the height of tensions.

Police guard a government building in Donetsk on February 26.

Protesters remove a fence that surrounds Ukraine’s parliament in Kiev on February 26.

People sing the Ukrainian national anthem at Independence Square on Monday, February 24.

Gas masks used by protesters sit next to a barricade in Independence Square on February 24.

A woman cries February 24 near a memorial for the people killed in Kiev.

People wave a large Ukrainian flag in Independence Square on Sunday, February 23.

Two pro-government supporters are made to pray February 23 in front of a shrine to dead anti-government protesters.

A man and his daughter lay flowers at a memorial for protesters killed in Independence Square.

Former Prime Minister Yulia Tymoshenko speaks at Independence Square on Saturday, February 22, hours after being released from prison. Tymoshenko, considered a hero of a 2004 revolution against Yanukovych, was released after 2½ years behind bars.

Tymoshenko is greeted by supporters shortly after being freed from prison in Kharkiv on February 22.

A protester guards the entrance to Yanukovych’s abandoned residence outside Kiev on February 22.

Anti-government protesters guard the streets next to the presidential offices in Kiev on February 22.

Anti-government protesters drive a military vehicle in Independence Square on February 22. Many protesters said they wouldn’t leave the square until Yanukovych resigned.

Ukrainian lawmakers argue during a session of Parliament on Friday, February 21.

Men in Kiev carry a casket containing the body of a protester killed in clashes with police.

Protesters cheer after news of an agreement between the opposing sides in Kiev on February 21.

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Have you witnessed the crisis in Ukraine, particularly in Crimea? Share your experiences, but please stay safe.

Kiev, Ukraine (CNN) — As Ukraine‘s new leaders accused Russia of declaring war, Russia’s Prime Minister warned Sunday that blood could be spilled amid growing instability in the neighboring nation.

Kiev mobilized troops and called up military reservists in a rapidly escalating crisis that has raised fears of a conflict. And world leaders pushed for a diplomatic solution.

In a post on his official Facebook page, Russian Prime Minister Dmitry Medvedev called the recent ouster of Ukrainian President Viktor Yanukovych a “seizure of power.”

“Such a state of order will be extremely unstable,” Medvedev said. “It will end with the new revolution. With new blood.”

Officials said signs of Russian military intervention in Ukraine’s Crimean peninsula were clear.

Russian generals led their troops to three bases in the region Sunday, demanding Ukrainian forces surrender and hand over their weapons, Vladislav Seleznyov, spokesman for the Crimean Media Center of the Ukrainian Defense Ministry, told CNN.

By late Sunday, Russian forces had “complete operational control of the Crimean Peninsula,” a senior U.S. administration official said. The United States estimates there are 6,000 Russian ground and naval forces in the region, the official said.

“There is no question that they are in an occupation position — flying in reinforcements and settling in,” another senior administration official said.

Speaking by phone, Seleznyov said Russian troops had blocked access to bases but added, “There is no open confrontation between Russian and Ukrainian military forces in Crimea” and said Ukrainian troops continue to protect and serve Ukraine.

“This is a red alert. This is not a threat. This is actually a declaration of war to my country,” Ukrainian interim Prime Minister Arseniy Yatsenyuk said.

Speaking in a televised address from the parliament building in the capital, Kiev, he called on Russian President Vladimir Putin to “pull back his military and stick to the international obligations.”

“We are on the brink of the disaster.”

A strange scene, somewhat polite standoff in Crimea

Kerry heading to Kiev

Ukraine PM: ‘This is a red alert’

On GPS: How will Ukraine respond?

Ukraine opposition leader speaks out

A sense of escalating crisis in Crimea — an autonomous region of eastern Ukraine with strong loyalty to neighboring Russia — swirled, with U.S. Secretary of State John Kerry condemning what he called Russia’s “incredible act of aggression.”

Speaking on the CBS program “Face The Nation,” Kerry — who is set to arrive in Kiev on Tuesday — said several foreign powers are looking at economic consequences if Russia does not withdraw its forces.

“All of them, every single one of them are prepared to go to the hilt in order to isolate Russia with respect to this invasion,” he said. “They’re prepared to put sanctions in place, they’re prepared to isolate Russia economically.”

Kerry rebukes Russia’s ‘incredible act of aggression’

But Ukraine’s ambassador to the United Nations said his country needs more than diplomatic assistance.

“We are to demonstrate that we have our own capacity to protect ourselves … and we are preparing to defend ourselves,” Yuriy Sergeyev said on CNN’s “State of the Union.” “And nationally, if aggravation is going in that way, when the Russian troops … are enlarging their quantity with every coming hour … we will ask for military support and other kinds of support.”

Pushing diplomatic possibilities

In Brussels, Belgium, NATO ambassadors held an emergency meeting on Ukraine.

“What Russia is doing now in Ukraine violates the principles of the U.N. charter,” NATO Secretary General Anders Fogh Rasmussen told reporters. He later added that Russia’s actions constituted a violation of international law.

He called upon Russia to honor its international commitments, to send it military forces back to Russian bases, and to refrain from any further interference in Ukraine.

Rasmussen also urged both sides to reach a peaceful resolution through diplomatic talks and suggested that international observers from the United Nations should be sent to Ukraine.

German Chancellor Angela Merkel’s office said Putin had accepted a proposal to establish a “fact-finding mission” to Ukraine, possibly under the leadership of the Organization for Security and Co-operation in Europe, and to start a political dialogue.

U.N. Secretary-General Ban Ki-moon dispatched a special envoy to Ukraine Sunday evening, a spokesman for his office said.

20 questions: What is Russia’s interest in Ukraine?

Lean to the West, or to Russia?

Ukraine, a nation of 45 million people sandwiched between Europe and Russia’s southwestern border, has been plunged into chaos since the ouster of President Viktor Yanukovych on February 22 following bloody street protests that left dozens dead and hundreds wounded.

Russia OKs military force in Ukraine

Kiev: Russia’s move is direct aggression

Anti-government protests started in late November when Yanukovych spurned a deal with the EU, favoring closer ties with Moscow instead.

Ukraine has faced a deepening split, with those in the west generally supporting the interim government and its European Union tilt, while many in the east prefer a Ukraine where Russia casts a long shadow.

Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.

Ukrainian leaders and commentators have compared events in Crimea to what happened in Georgia in 2008. Then, cross-border tensions with Russia exploded into a five-day conflict that saw Russian tanks and troops pour into the breakaway territories of South Ossetia and Abkhazia, as well as Georgian cities. Russia and Georgia each blamed the other for starting the conflict.

By Sunday night, electricity had been cut off at the headquarters of the Ukrainian Navy in Crimea, and officials feared there could soon be an attack, Seleznyov said.

CNN has not independently verified that claim, and Russian officials could not be immediately reached to respond.

Cold War-style conflict hits Crimea: 3 things to know

Military maneuvering

Word of the power outage came hours after the newly named head of Ukraine’s navy disavowed Ukraine’s new leaders and declared his loyalty to the pro-Russian, autonomous Crimea government.

Rear Adm. Denis Berezovsky, who was appointed Saturday by interim Ukrainian President Oleksandr Turchynov, said from Sevastopol on the Black Sea that he will not submit to any orders from Kiev.

He was quickly suspended and replaced by another rear admiral, the Defense Ministry in Kiev said in a written statement.

These scenes come one day after Putin obtained permission from his parliament to use military force to protect Russian citizens in Ukraine, spurning Western pleas not to intervene.

Putin cited in his request a threat posed to Russian citizens and military personnel based in southern Crimea.

Ukrainian officials have vehemently denied Putin’s claim.

Opinion: Putin’s move could be costly to U.S., Middle East

Western governments worried

The crisis set off alarm bells in the West and fueled a stern rebuke from the leaders of the G7 nations of Canada, France, Germany, Italy, Japan, the United Kingdom and the United States.

In a statement Sunday, they condemned Russia’s “clear violation of the sovereignty and territorial integrity of Ukraine,” saying they were temporarily suspending activities related to preparation for June’s G8 Summit in Sochi, Russia.

Canada recalled its ambassador to Moscow.

Senior Obama administration officials Sunday portrayed Russia’s intervention in Ukraine as weak, describing it in a conference call with reporters as a kind of desperate measure from a man who realizes he has lost support of the international community.

When asked what concrete measures the administration has taken to signal its strong opposition to Russian involvement in Ukraine, the officials noted that planning meetings about the upcoming G8 summit in Sochi had been canceled. In the long term, economic sanctions could be employed, they said. The officials declined to be more specific about what those sanctions might involve.

In discussions over the weekend with Putin, Obama “made clear that Russia’s continued violation of Ukraine’s sovereignty and territorial integrity would negatively impact Russia’s standing in the international community,” according to a statement released by the White House.

During that call, one administration official said, Putin did not “slam the door” to the idea that international monitors could travel to Ukraine to make sure violence doesn’t flare up, one official said.

According to the Kremlin, Putin told Obama that Russia reserves the right to defend its interests in the Crimea region and the Russian-speaking people who live there.

Obama met Sunday with his national security team and called U.S. allies afterward, White House spokesman Josh Earnest said.

British Prime Minister David Cameron said he and Obama were of the same mind when they spoke on Sunday.

“We agreed Russia’s actions are unacceptable and there must be significant costs if they don’t change course,” Cameron posted on his verified Twitter account.

Cameron also planned to talk with Lithuanian President Dalia Grybauskaite and Polish Prime Minister Donald Tusk.

Britain’s Foreign Minister William Hague arrived Sunday in Kiev, where he will meet with Ukraine leaders.

Diplomatic language on Ukraine is short on specifics

CNN’s Ben Wedeman, Victoria Eastwood and Diana Magnay reported from Simferopol, Ukraine; Ian Lee, Ingrid Formanek and Victoria Butenko from Kiev, while Marie-Louise Gumuchian wrote from London and Steve Almasy and Catherine E. Shoichet wrote from Atlanta. CNN’s Chelsea J. Carter, Bharati Naik, Richard Roth, Laura Smith-Spark, Tom Watkins, Sara Mazloumsaki, Alla Eshchenko, Arkady Irshenko, Radina Gigova, Kevin Liptak, Karen Smith and Ashley Fantz and journalists Azad Safarov and Elena Sandryev contributed to this report.