"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. "And I think the strong condemnation that it's received from countries around the world indicates the degree to which Russia's on the wrong side of history on this."
"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.