Russian President Vladimir Putin has got the go-ahead from parliament to use the country’s military in Ukraine in a marked escalation of the crisis.
The Kremlin has already been accused of sending 6,000 troops into Crimea despite calls by Britain and the US for Moscow to back off.
On Friday, US President Barack Obama warned Moscow “there will be costs” if it intervened militarily.
France and Germany have also raised concerns over the developments.
Mr Putin said the use of armed forces was needed in the southeastern region to protect its majority ethnic Russian population, and personnel of Russia’s Black Sea Fleet based there.
In response, Ukraine’s Acting President Oleksandr Turchynov has called an urgent meeting of security chiefs.
European foreign ministers are also to hold emergency talks in Brussels on Monday.
British Foreign Secretary William Hague said he was “deeply concerned” at the escalation of tensions and the decision of the Russian parliament to authorise military action.
“This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine,” he said.
Mr Hague is due to visit the country on Sunday and hold talks with Ukraine’s new leaders.
Sky’s Alex Rossi, in Simferopol, Crimea, said: “It has been a very fluid day of developments. It seems there are thousands of unidentified troops on the streets here in the Crimean peninsula.
“The people that we’ve spoken to, around the parliament building, they welcome those troops, seeing them as liberators – liberating this region from the tyrannical government that is now in place in Kiev.
“That new unity government that was only voted in this week, though, sees it very differently indeed…. see more