Ukraine accuses Russia of deploying troops in Crimea
Ukraine's acting President Oleksander Turchynov has accused Russia of deploying troops to Crimea and trying to provoke Kiev into "armed conflict".
In a TV address, he said Moscow wanted the new interim government to react to provocations so it could annex Crimea.
Russia's UN ambassador said any troop movements in Crimea were within an existing arrangement with Ukraine.
US President Barack Obama warned of the "costs" of any Russian intervention in the Ukraine.
In a statement from the White House he said: "Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilising, which is not in the interests of Ukraine, Russia or Europe."
He said Washington stood by Ukraine's new interim government and commended "its restraint and its commitment to uphold its international obligations".
President Turchynov appealed to Russian President Vladimir Putin to "stop provocations and start negotiations".
He said Russia was behaving as it did before sending troops into Georgia in 2008 over the breakaway regions of Abkhazia and South Ossetia, which have large ethnic Russian populations.
"They are implementing the scenario like the one carried out in Abkhazia, when after provoking a conflict, they started an annexation of the territory," President Turchynov said.
Protecting 'strategic sites'
His statement came a few hours after the Kremlin said President Putin had spoken of the "extreme importance of not allowing a further escalation of violence" during telephone conversations with Western leaders.
However, flights from and to the Crimean capital, Simferopol, were cancelled with airlines saying airspace over the peninsula had been closed.
Senior Ukrainian official Sergiy Kunitsyn told local media 13 Russian aircraft carrying nearly 2,000 suspected troops had landed at a military air base near Simferopol. This remains unconfirmed.
Russia's ambassador to the UN, Vitaly Churkin, said that any Russian military movements in Crimea were within Moscow's long-standing arrangement with Ukraine on the deployment of military assets.
"We are acting within the framework of that agreement," he said, after a closed-door meeting of the UN Security Council.
He did not give details of any Russian military deployment.
Earlier in the day, Russian armoured vehicles and helicopters were seen in and around Simferopol and Sevastopol, where Russia's Black Sea Fleet are based.
Armed men in unidentified military uniforms, believed to be loyal to Russia, have moved in on Crimea's parliament, state television building and telecommunication centres.
They are also patrolling the airports in Simferopol and Sevastopol.
The editor of the main television station told the BBC he was informed the men were protecting strategic points in the area, in this case the broadcasting tower.
Ukraine has formally lodged a protest with Russia over what it called a violation of Ukrainian airspace and a failure to observe an agreement on the stationing of Russia troops in Crimea.
On Friday, Ukraine's former President Viktor Yanukovych made his first public appearance since being ousted from office a week ago.
At a news conference in Russia, he apologised for not "having enough strength to keep stability" in Ukraine and called his usurpers "young, neo-fascist thugs".
Mr Yanukovych said he would "continue to struggle for the future of Ukraine", but said he would only return if his safety could be guaranteed.
Ukraine has started procedures demanding his extradition.
He is wanted on suspicion of mass murder following violent clashes between police and protesters last week that left more than 80 dead.
Kiev had been in political deadlock since last November when Mr Yanukovych rejected a trade deal with the EU in favour of a similar agreement with Russia.
The move brought thousands of Western-leaning protesters out on to the streets calling for his resignation and closer ties with the EU.
Since he was deposed, the tensions have shifted to Crimea, where the majority of the population are ethnic Russians.