Russian President Vladimir Putin seemed unmoved by comments from other world leaders.
Russian troops consolidated their hold on Ukraine's Crimean peninsula as the United States and others denounced the military movement, deepening the East-West standoff over the future of Ukraine.
U.S. Secretary of State John Kerry and European foreign ministers announced plans Sunday for talks in Kiev aimed at easing rising tensions as Ukraine put its military on high alert and appealed for international help against a feared invasion by Russia.
Ukrainian Prime Minister Arseniy Yatsenyuk said his country "was on the brink of national disaster." The interim government in Kiev said the country's military should be ready to fight. Ukrainian military reservists were ordered to active duty while in the Crimean region in the south of the country, road traffic was blocked and telecommunications remained sporadic — two days after communication centers were seized by unknown armed men.
Earlier Sunday, hundreds of armed men in trucks and armored vehicles surrounded a Ukrainian military base in Crimea, blocking its soldiers from leaving in a standoff when Ukrainians placed a tank at the base's gate.
"The situation is very serious — the Russian army is blocking military bases of Ukraine in Crimea. … They issued an ultimatum demanding that our soldiers disarm themselves or the bases will be stormed," said Ukraine's interim president and parliament speaker, Oleksandr Turchinov, on Sunday.
President Obama met with his national security team and called international leaders in an effort to coordinate a response. He spoke Sunday with German Chancellor Angela Merkel, British Prime Minister David Cameron and Polish President Bronislaw Maria Komorowski.
Kerry called the occupation "an incredible act of aggression."
American allies "are prepared to go to the hilt in order to isolate Russia with respect to this invasion," Kerry said on CBS' Face The Nation. "They're prepared to put sanctions in place, they're prepared to isolate Russia economically."
The United States on Saturday suspended its participation in the G-8 talks among the world's largest economic powers. The talks are scheduled to be held in Sochi, Russia in June.
PUTIN, OBAMA DISAGREE
Russian President Vladimir Putin seemed unmoved by comments from other world leaders. Putin told Obama in a 90-minute telephone conversation late Saturday night, that use of force on Russia's part would be a response to provocations from Ukraine.
In that phone call and his calls with other leaders, Obama said the Russian actions in Ukraine lacked any international authority, according to three senior administration officials who briefed reporters Sunday afternoon.
"The president's point in all of his calls is to point out the complete illegitimacy of Russia's intervention in the Crimea and Ukraine," said one of the officials, all of whom spoke on the condition of anonymity to discuss the sensitive diplomatic efforts.
Putin told Obama that not only can Russia send its troops to Crimea, but to all of predominantly Russian-speaking eastern Ukraine due to "the existence of real threats" to Russian citizens in Ukrainian territory, according to a statement on the Kremlin's website.
A second senior administration official said Russia has moved 6,000 troops into the Crimean peninsula, and that there are signs of tensions being "stirred up" in eastern Ukraine between ethnic Russians and Ukrainians. The administration declined to assign blame to a particular person or party for the tension.
RUSSIANS TAKING RISK
Crimea is a semi-autonomous region of Ukraine with a majority ethnic Russia presence, many of whom have openly welcomed the presence of Russian troops. It is home to critical Russian military bases and access to the Black Sea.
But Russia risks getting sucked into a quagmire if Moscow attempts to expand further into Ukraine, analysts said.
"It's not going to be the short five- or six-day war they had with Georgia," said Steven Pifer, a former U.S. ambassador to Ukraine now a senior fellow at Brookings Institution. Russia went to war in 2008 with Georgia over a territorial dispute.
Any attempt by Russia's forces to push beyond Crimea and further into Ukraine would risk partisan warfare, terror attacks and resistance from Ukraine's army, security analysts say. But they disagreed about what Russia may do next. It could remain in Crimea, expand its reach into other parts of the Ukraine or begin withdrawing.
"Unless there is a strong response from the West -- and I mean strong response -- they will keep moving," said Stephen Blank, a senior fellow with the American Foreign Policy Council. "They've bet the farm."
Entering other parts of Ukraine, however, would be far more difficult for Russian forces and Russia's interests there are less clear. Russia would meet resistance from the public and Ukrainian forces. Even eastern Ukraine, where there is a significant ethnic Russian presence, Russian forces would receive a mixed reception at best, analysts say.
In those regions they would likely face guerrilla warfare and attacks on infrastructure, including a pipeline that carries Russian natural gas to Europe, said Adrian Karatnycky, an analyst with the Atlantic Council. "It would be a bloody mess," he said.
Russia has a large army, but much of it consists of poorly trained conscripts. Its forces could probably overrun Ukraine's military, which consists of about 130,000 troops, but trying to occupy a large portions of Ukraine would be costly and difficult.
Analysts believe Putin is weighing these practical considerations as much as the international response as he decides his next step. "Putin is pushing the boundaries," said Alisa Moldavanova, a political science professor at Wayne State University who has researched Ukrainian security issues. "He is trying to see how far he can go."
A more practical strategy for Russia would be to keep Ukraine weak by pushing for Crimea's independence and encouraging a movement within Ukraine to create a semi-autonomous region in the east. "If anything they might push for the federalization of Ukraine," Moldavanova said.
REPUBLICANS FAULT OBAMA
In Washington, some Republicans used the crisis to fault Obama's own foreign policy.
"Putin is playing chess and I think we're playing marbles," said Rep. Mike Rogers, R-Mich., chairman of the House Intelligence Committee, speaking on Fox News Sunday. Rogers said the Russians have been "running circles around us" in negotiations on such items as Syria and missile defense.
Sen. Lindsey Graham, R-S.C., speaking on CNN's State of the Union, said "we have a weak and indecisive president," and that "invites aggression."
Rogers, Graham and other officials who appeared on Sunday interview shows supported calls for new sanctions, and suggested that Russia be kicked out of the G-8. They also called for economic assistance to Ukraine and help for other developing democracies located near Russia.
Resneck reported from Sevastopol, Ukraine; Michaels from Washington.
Contributing: Olga Rudenko and Jabeen Bhatti; Michaels, Aamer Madhani and David Jackson from Washington; The Associated Press