The United States has demanded an end to Russian intervention in Ukraine.
It's UN Ambassador Samantha Power described the Russian decision to send troops into Ukraine as "as dangerous as it is destabilising".
It comes as Barack Obama phone Russian president Vladimir Putin to discuss the crisis.
Russia's President Vladimir Putin has approval from Russia's upper house of parliament, to send Russian troops into Ukrainian territory.
Two Russian warships have reportedly appeared off the Ukrainian coast near Sevastopol - while Russian MPs have approved sending troops into the country.
In Kiev, the interim administration's warned Moscow any military intervention would lead to war.
Sky's Europe correspondent Robert Nisbet says things are on a knife-edge.
EU foreign ministers are to hold an emergency meeting in Brussels on Monday over the continuing crisis in Ukraine.
UN Secretary General Ban Ki-moon says he's 'gravely concerned' about the situation in Ukraine, following reports two Russian warships have appeared off the Ukrainian coast near Sevastopol.
Ukraine's interim government says six thousand Russian troops have already entered the Crimea region - despite calls from Britain and America for Russia to back off.
The acting President of Ukraine, Oleksander Turchinov, claims Russia is trying to destabilise his country and has also warned Russia any military intervention would lead to war and end relations with Moscow.
Andy Hunder - who's director of the Ukrainian Institute - says President Putin wants to expand his empire.
The pro-Moscow leader of the autonomous Ukraine region of Crimea had called on Vladimir Putin to help ensure “peace and tranquillity” in Crimea.
A Kremlin source, speaking to the BBC, has said the Russian government would “not leave unnoticed” the request.
Sergiy Aksyonov – leader of Crimea’s ruling party since an emergency vote earlier this week – said:"I appeal to the president of the Russian Federation, Vladimir Putin, to provide assistance in ensuring peace and tranquility on the territory of the Autonomous Republic of Crimea."
Aksyonov's statement follows from yesterday's escalation in events when mysterious groups of armed men occupied several areas of Simferopol, Crimea's main city, including the largest commercial airport. Military trucks and armoured vehicles also blocked roads. Reports claimed the men, wearing no national insignia but carrying Russian standard equipment, were Russian military forces.
The election of Aksyonov has not been approved by the new government in Kiev.
Aksyonov went further, announcing he was taking charge of Crimea’s military forces.
"All commanders are to obey only my orders and instructions.
"I ask all those who refuse to do so to resign,” Aksyonov said.
New Ukrainian Prime Minister Arseniy Yatsenyuk said: "The presence of Russian soldiers is a provocation and we demand that Russian soldiers return to their permanent bases."
President Barack Obama has warned Russia that there will be “costs” if Russian troops intervenes militarily in Ukraine.
