"Beginning from 24 February, approximately 16,000 Russian troops have been deployed in Crimea by the military ships, helicopters, cargo airplanes from the neighboring territory of the Russian Federation," Sergeyev told an emergency meeting of the UN Security Council on the crisis in his country.

Russia has violated international law in Ukraine: Obama

President Barack Obama said on Monday that Russia has violated international law with its military intervention in Ukraine and warned that US government will look at a series of economic and diplomatic sanctions that would isolate Moscow.

Russian President Vladimir Putin needs to allow international monitors to mediate a deal in Ukraine acceptable to all Ukrainian people, Obama told reporters before he met with Israeli Prime Minister Benjamin Netanyahu.

US is weighing its response to Russia's so far bloodless incursion into Crimea. Despite an international outcry, Putin has shown little sign of backing down, and Russia has built up armored vehicle presence near Crimea and staged military maneuvers in what appears to be a show of strength.

"Over time this will be a costly proposition for Russia. And now is 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.

US Secretary of State John Kerry will propose ways in which a negotiation between Russia and Ukraine could be overseen by a multilateral organization when he goes to Kiev on Tuesday, Obama said. Kerry will also offer an economic aid package for Ukraine, the president added.

In the meantime, the United States is preparing to impose sanctions on Russia over the intervention although no decisions have yet been made, the State Department said. State Department spokeswoman Jen Psaki said United States has a broad range of options.

In her daily briefing, Psaki also said that if reports that Russia has threatened Ukraine's military in the Crimea with attack are true, this would be a "dangerous escalation" of the situation.

As the administration sought to rally international pressure against Russia, US lawmakers were considering imposing sanctions on Russia's banks and freezing assets of Russian public institutions and private investors in response to events in Ukraine, one US Senator said.

The lawmaker, Democrat Chris Murphy, warned that in order to be effective, US sanctions would need to be matched by similar moves by Europe.

"If United States shuts its economic doors to Russia and Europe leaves its doors open, there won't be much change in behavior from Moscow," he said.

Obama has been criticized by opposition party Republicans for not being strong enough in his response to the Ukraine crisis.

The president called on lawmakers to unite behind an aid package for the embattled eastern European country.

"When it comes to preserving the principle that no country has the right to send in troops to another country unprovoked, we should be able to come up with a unified position," he said.

At the Obama administration's request, members of both the Senate and House of Representatives are working on legislation to support US loan guarantees for Ukraine, congressional aides said.


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