(CNN) -- The United Kingdom will suspend participation in preparatory talks for the G8 meeting in Sochi, Russia, Great Britain's Foreign Secretary William Hague told Reuters on Sunday. According to media reports citing a source in President Francois Hollande's office, France said it also will not participate in the talks. The G8 meeting is scheduled for June 4 and 5.
From CNN's Jennifer Deaton
KIEV, Ukraine (CNN) -- Ukraine's new Prime Minister Arseniy Yatsenyuk warned Sunday his crisis-hit country was on the "brink of disaster," accusing neighbor Russia of declaring war.
Ukraine's shaky new government mobilized troops and called up military reservists Sunday, even as the defense minister said Kiev stood no chance against Russian troops in a rapidly escalating crisis that has raised fears of a conflict.
Amid signs of Russian military intervention in Ukraine's Crimean peninsula, 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.
Speaking by phone, he said Russian troops had blocked access to the bases, but added "there is no open confrontation between Russian and Ukrainian military forces in Crimea" and that Ukrainian troops continue to protect and serve Ukraine.
"This is a red alert. This is actually a declaration of war in our country," Yatsenyuk said.
Speaking in a televised address from the parliament building in Kiev, Yatsenyuk called on Russian President Vladimir Putin to "pull back the military and stick to international obligations."
"We are on the brink of disaster," he said.
In Brussels, Belgium, NATO ambassadors were scheduled to hold 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.
"Russia must stop its military activities and threats," Rasmussen said, adding, "we support Ukraine's territorial integrity and sovereignty. ... We support the rights of the people of Ukraine to determine their own future without outside interference."
Ukraine's parliament met behind closed doors Sunday. At the closing of the session, acting Defense Minister Ihor Tenyuh said Ukraine does not have the military force to resist Russia, according to two parliamentary members present at the meeting. Tenyuh called for talks to resolve the crisis with Russia, they said.
The Ukrainian National Security Council has ordered the mobilization of troops, as Putin appeared to dismiss warnings from world leaders to avoid military intervention in Crimea, a senior Ukrainian official, Andriy Parubiy, said.
He also said the Defense Ministry was calling for reservists to register at the local and regional headquarters to be on standby if needed.
A sense of escalating crisis in Crimea -- an autonomous region of eastern Ukraine with strong loyalty to neighboring Russia -- swirled Saturday night, with U.S. Secretary of State John Kerry condemning what he called "the Russian Federation's invasion and occupation of Ukrainian territory."
Russia has not confirmed it deployed thousands of troops to the region following reports that armed, Russian-speaking forces wearing military uniforms -- without insignia -- patrolled key infrastructure sites.
It was the latest in a series of fast-moving developments that saw Russia's parliament sign off on Putin's request to send military forces into Ukraine, raising the stakes in the escalating brinksmanship. Putin cited in his request a threat posed to the lives of Russian citizens and military personnel based in southern Crimea. Ukrainian officials have vehemently denied Putin's claim.
According to a tweet from the official Russian government account Sunday, Russian Prime Minister Dmitry Medvedev discussed the crisis in Ukraine in a telephone call with Yatsenyuk.
According to a second tweet, Medvedev said Russia is interested in maintaining stable and friendly relations with Ukraine but reserves the right to protect the legitimate interests of its citizens and military personnel stationed in Crimea.
Path to war?
In Kiev, thousands of people rallied in the central Independence Square, cradle of Ukraine's three-month anti-government protests that led to President Viktor Yanukovych's ouster last week.
A crowd held up signs reading "Crimea, we are with you" and "Putin, hands off Ukraine."
In Moscow, about 50 protesters were detained outside a Defense Ministry building, a Moscow police spokesman said.
Putin's move prompted world diplomats to call for a de-escalation of tensions that have put the two neighbors on a possible
path to war and roiled relations between Russia and the United States.
In what appeared to be an illustration of the growing schism between the two world powers, U.S. President Barack Obama and Putin spoke for 90 minutes, with each expressing his concern over the mounting crisis, according to separate statements released by their governments.
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.
The Russian government said in a statement that, in reply to U.S. concerns over the possibility of the use of Russian armed forces in Ukraine, Putin "drew his attention to the provocative and criminal actions on the part of ultranationalists who are in fact being supported by the current authorities in Kiev."
According to a statement released Saturday by the White House, 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."
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 a week ago of Yanukovych following bloody street protests that left dozens dead and hundreds wounded.
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.
Ukraine's acting President Oleksandr Turchynov took to the airwaves late Saturday to warn that any Russian military intervention would lead to war.
'The troops are already there'
The crisis set off alarm bells with the world's diplomats, with Ukrainian Ambassador to the U.N. Yuriy Sergeyev calling on member nations of the Security Council to take a stand against what he called Russia's "clear act of aggression.''
"The troops are already there, and their number is increasing every hour," Sergeyev said during an emergency meeting of the Security Council.
Russia now has 15,000 troops Crimea, Yegor Pyvovarov, the spokesman for the Ukrainian mission at the United Nations, told CNN ahead of Saturday's session of the Security Council. He did not say how Ukraine arrived at that number, or whether that included troops already stationed at a Russian base in the region.
Vitaly Churkin, Russia's ambassador to the United Nations, rejected Ukraine's calls to stop Russian intervention. "We can't agree with this at all," he said.
He blamed members of the European Union for causing the bloody street demonstrations in Ukraine.
"It's a difficult situation in the past few hours," Churkin said, claiming that there were Ukrainian forces from Kiev en route to to overthrow the local pro-Russian governments in eastern Ukraine and Crimea and establish new ones that would enforce the power of the new Ukrainian government.
Churkin has said reports of Russian troops taking charge of positions on the ground were rumors and noted that rumors "are always not true."
Crimea's pro-Russian leader asked for help
The Russian parliament vote Saturday came on the day that the newly installed pro-Russian leader of Crimea, Sergey Aksyonov, asked Putin for help in maintaining peace on the Black Sea peninsula, where Russia's fleet is based at Sevastopol.
Security forces "are unable to efficiently control the situation in the republic," he said in comments broadcast on Russian state channel Russia 24. Aksyonov was installed as the region's premier after armed men took over the Crimean parliament building on Thursday.
Aksyonov said that a referendum on greater Crimean autonomy, originally set for May 25, would be moved to March 30.
Yatsenyuk called the Russian presence in Crimea a provocation.
"Ukraine will not be provoked, we will not use force. We demand that the government of the Russian Federation immediately withdraw its troops and return to their home bases," he said during a televised Cabinet meeting.
Airspace in the region reopened Saturday, a day after Ukraine accused Russian Black Sea forces of trying to seize two airports in Crimea but said Ukrainian security forces had prevented them from taking control.
Groups of armed men, dressed in uniforms without identifying insignia, patrolled the airports in Simferopol and the nearby port city of Sevastopol. The men remained at the airports Saturday, but Yevgey Plaksin, director of the airport in Simferopol, said airport services were working.
Obama: Warning to Russia
Senior White House officials say they are looking at a wide range of possible economic and diplomatic measures to present to Obama that would show Putin there is a cost to his actions in
The White House has already announced the United States will suspend participation in preparatory meetings for the G-8 summit that will bring world leaders together in June in Sochi, Russia.
"Going forward, Russia's continued violation of international law will lead to greater political and economic isolation," according to a statement released by the administration.
Pressure has been mounting on Russia as leaders from the EU and the UK joined an international outcry, with EU High Representative Catherine Ashton deploring Russia's "unwarranted escalation of tensions." British Foreign Secretary William Hague was scheduled to travel to Kiev on Sunday.
During a telephone call with Putin, U.N. Secretary General Ban Ki-Moon said he told the Russian leader that it was crucial to "restore calm and proceed to an immediate de-escalation of the situation."
"Cool heads must prevail and dialogue must be the only tool in ending this crisis," he said.
CNN's Victoria Eastwood and Diana Magnay reported from Simferopol, Ukraine; Ian Lee, Ingrid Formanek and Victoria Butenko from Kiev, while Marie-Louise Gumuchian and Chelsea J. Carter wrote from Atlanta. CNN's Richard Roth, Laura Smith-Spark, Tom Watkins, Sara Mazloumsaki, Alla Eshchenko, Arkady Irshenko, Radina Gigova and journalist Azad Safarov contributed to this report.
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