The United States has demanded an immediate end to Russian intervention in Ukraine as Kiev urged the United Nations Security Council to take immediate action to stop the crisis.
After the Russian parliament endorsed military action in Ukraine and Kiev warned Russian troops were pouring into Crimea, the 15-member Security Council convened emergency talks in New York on Saturday.
The US ambassador to the United Nations, Samantha Power, branded Russia's parliamentary approval "as dangerous as it is destabilising" and accused Moscow of acting without legal basis.
"It violates Russia's commitment to protect the sovereignty territorial integrity and independence of Ukraine," she said.
"It is time for the Russian intervention in Ukraine to end. The Russian military must stand down."
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Power called for monitors from the UN and the Organization for Security and Co-operation in Europe to be deployed to Ukraine and backed a separate international mediation mission to Crimea.
The monitors would "provide transparency about the movement and activities of military and paramilitary forces in the region, and diffuse the tensions between different groups", Power said.
"Our paramount concerns are to end the confrontation and to find a solution that allows the Ukrainian people to determine their own destiny, their own government, their own future."
The open session of the Security Council, which only got underway after two hours of debate on whether to allow the cameras in or not, began with an emotional appeal from Kiev.
"We call on the Security Council now to do everything possible to stop aggression of the Russian Federation to Ukraine. There is still a chance," Ukrainian Ambassador Yuriy Sergeyev said.
He accused Moscow of aggression against Ukraine and claimed that more Russian troops were arriving by the hour.
Ukraine also demanded the deployment of international monitors and said Russia had "brutally violated the basic principles of the charter of the United Nations".
"We urge all member states of the United Nations to demonstrate solidarity with the Ukrainian nation to protect the sovereignty and territorial integrity of the country," Sergeyev added.
Ukrainian Defence Minister Igor Tenyukh accused Russia of sending 30 armoured personnel carriers and 6000 additional troops into Crimea.
Dozens of pro-Russian armed men in full combat gear patrolled outside the seat of power in Crimea's capital Simferopol.
Similar gunmen seized the city's parliament and government buildings on Thursday and took control of its airport and a nearby military base on Friday.
The Security Council spent nearly two hours debating British, French and US demands supporting Ukraine's request that the session be public, which Russia initially opposed.
UN Secretary-General Ban Ki-moon called for an immediate restitution of calm in Ukraine and expressed "grave" concerns directly to Russian leader Vladimir Putin by telephone.
"It is important to recall the mission of this organisation, to always search for peaceful settlements of dispute," deputy secretary general Jan Eliason told the Security Council.
"Now is the time for cool heads to prevail."
Russia's envoy criticised Western support for the protesters that ousted Russian-backed leader Viktor Yanukovych and said Putin had not yet decided to use force.
"I repeat, as Mr Eliason said quite correctly, we need cooler heads to prevail," ambassador Vitaly Churkin said.
