Obama said Russia has violated Ukraine's sovereignty and international law, and he warned Russian President Vladimir Putin to change course.
"Over time this will be a costly proposition for Russia and now's the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force," Obama said from the Oval Office. He spoke at the start of a meeting with Israeli Prime Minister Benjamin Netanyahu.
Earlier Monday, a Ukrainian military spokesman said Russia had issued an ultimatum to the crews of two Ukrainian warships in Crimea, demanding that they immediately surrender or be stormed and seized. Vladimir Anikin, a Russian defense ministry spokesman in Moscow, dismissed that report as nonsense, but he refused to elaborate.
Secretary of State John Kerry was leaving for Ukraine late Monday and then will travel to France and Italy. He had planned to see his Russian counterpart in Paris, but a spokeswoman said that meeting was no longer certain.
The spokeswoman, Jen Psaki, said any Russian threat to Ukraine's navy would be a "dangerous escalation" of an extremely tense situation, although she said she could not confirm if Russia had in fact made such threats.
The U.S. and its allies are weighing sanctions on Moscow, in what amounts to a sudden reprise of Cold War sensibilities. One consideration is whether to bolster defenses in Europe in response to Russia's military advances on Ukraine.
"What cannot be done is for Russia with impunity to put its soldiers on the ground and to violate basic principles that are recognized around the world," Obama said.
"What we are also indicating to the Russians is that if in fact they continue on the current trajectory that they're on, that we are examining a whole series of steps—economic, diplomatic—that will isolate Russia and will have a negative impact on Russia's economy and its status in the world," Obama said.
Putin gave no indication that he would heed the West's warnings. Hundreds of armed men surrounded a Ukrainian military base in Crimea, a pro-Russian area. In Kiev, Ukraine's capital, Prime Minister Arseniy Yatsenyuk warned that "we are on the brink of disaster."
"This is absolutely the most serious test of our alliances since the Cold War ended," Rep. Marcy Kaptur, D-Ohio, said in a nationally broadcast interview Monday.
"I think it is extremely dangerous. Ukrainians fight and Russians fight," said Kaptur, who has traveled to Ukraine on several occasions and is considered an expert on that part of the world.
Senior Obama administration officials said they believe Russia now has complete operational control over Crimea and has more than 6,000 forces in the region. The U.S. was also watching for ethnic skirmishes in other areas of eastern Ukraine, though the officials said they had not yet seen Russian military moves elsewhere. The officials were not authorized to publicly discuss the situation and spoke on condition of anonymity.
Treasury Secretary Jacob Lew said the United States is ready to work with other countries and the International Monetary Fund to provide support for Ukraine's economy.
In Brussels, NATO's secretary-general, Anders Fogh Rasmussen, said Russia's actions have violated a U.N. charter. He said the alliance was re-evaluating its relationship with Russia.
Beyond economic sanctions and visa bans, freezing Russian assets, and trade and investment penalties, Kerry said Moscow risks being booted out of the powerful Group of Eight group of world powers as payback for the military incursion.
Former Ambassador Nicholas Burns said, "Putin's not going to back off. ... What can President Obama do? Be very minded in opposition. We can't follow a military policy. This has to be diplomatic."
Several U.S. senators also called for bolstered missile defense systems based in Poland and the Czech Republic.
Russia is "going to be inviting major difficulties for the long term," said Kerry. "The people of Ukraine will not sit still for this. They know how to fight."
Sen. Marco Rubio, R-Fla., discussing the potential of U.S. military strikes against Russian troops in Crimea, said, "I don't think anyone is advocating for that." One of the administration officials indicated that the U.S. was not weighing military action to counter Russia's advances, saying the Obama administration's efforts were focused on political, economic and diplomatic options.
Rubio and fellow GOP Sen. Lindsey Graham of South Carolina said the Obama administration should return to plans it abandoned in 2009 to place long-range missile interceptors and radar in Poland and the Czech Republic.
Russia suggested the program was aimed at countering its own missiles and undermining its nuclear deterrent. The White House denied that and has worked instead to place medium-range interceptors in Poland and Romania—aimed at stopping missiles from Iran and North Korea.
AP White House Correspondent Julie Pace contributed to this report.
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