According to reports in Ukrainian media, Russia issued an ultimatum on Monday that all Ukrainian military forces in Crimea must surrender by 5:00 a.m. (10 p.m. EST) or face immediate military intervention. Earlier in the day, reports surfaced that Russia's military issued a similar ultimatum to three Ukrainian warships stationed in the Black Sea. Russia has denied both claims, which a spokesman in Moscow described as "ridiculous."

News of the instability in Ukraine sent stock markets plummeting; the Moscow stock market fell 11.3 percent, and the Dow Jones lost more 153 points.

In a statement, President Obama issued stern-sounding, if vague, threats of action if Russia refuses to stand down. "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 standing in the world," he said.

During an emergency session in Brussels, the European Union condemned Russia, promising "targeted measures" unless Russia retreats, though no consensus on a response was reached. From the New York Times:

The German foreign minister, Frank-Walter Steinmeier, said that "crisis diplomacy is not a weakness but it will be more important than ever to not fall into the abyss of military escalation," an indication that the Europeans would not agree on significant action. Dutch Foreign Minister Frans Timmermans told reporters that "sanctions are not in order today but sanctions will become inevitable" if there is no change in Russia's position.

Meanwhile, British Foreign Secretary William Hague visited Kiev to meet with and show support for the new Ukrainian government. U.S. Secretary of State John Kerry is set to meet him there tomorrow.

"The world cannot just allow this to happen," Hague told the BBC from Kiev. "The world cannot say it's O.K. in effect to violate the sovereignty of another nation in this way."

[Image via AP]