Russia refutes NATO allegations

"Those who try to interpret the situation as a type of aggression and threaten sanctions and boycotts, are the same who consistently have encouraged (Ukrainians to) refuse dialogue and have ultimately polarized Ukrainian society," Lavrov told the meeting.

Ukrainian authorities said the Russian troops had issued an ultimatum for two of the country's besieged warships to surrender and deployed 16,000 troops in Crimea.

In response, the US Defense Department said it was suspending exercises and other activities with the Russian military after the Obama administration was examining "a whole series of" economic and diplomatic steps to "isolate" Russia.

The European Union (EU) also mulled "targeted measures" against Russia due to the situation in Ukraine.

"In the absence of de-escalating steps by Russia, the EU shall decide about consequences for bilateral relations between the EU and Russia, for instance suspending bilateral talks with Russia on visa matters as well as on the New Agreement, and will consider further targeted measures," EU foreign policy chief Catherine Ashton told a press conference Monday after an extraordinary EU foreign ministers' meeting.

Global market reacted furiously to the Ukraine crisis. On Wall Street, both the Dow Jones industrial average and the Nasdaq composite closed sharply down, while oil prices rose on fears that Russia, a major oil exporter, might face sanctions. In European trading, gold rose while the euro and stock markets fell.

Phone call puts Obama, Putin in confrontation	Ukraine mobilizes after Putin's move