Obama warns Putin over Ukraine

US President Barack Obama has warned Russia that military intervention in Ukraine would lead to "costs," as tension with old foe President Vladimir Putin rose in a Cold War-style crisis.

Speaking on Friday, Obama said he was "deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine", after the ouster of Ukrainian leader Viktor Yanukovich.

"The US will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," Obama said in the White House briefing room.

Tensions between the Cold War rivals soared after reports that 2,000 troops were airborne to a military airbase near the regional capital of the restive Crimean peninsula, which Ukraine's central government deems an occupation and military aggression.

Government buildings, including two airports, in the semi-autonomous Crimea region, which has a majority of ethnic Russians, have had the Russian navy flag marking their roofs after pro-Kremlin armed men took control late Thursday.

Obama and other world leaders are looking into "possible repercussions" if Russia were to militarily intervene in Ukraine, a former member of the dissolved Soviet union.

But as Russia denies any wrongdoing, the crisis is unlikely to ease.

"You all know we have an agreement with Ukraine on the presence of the Russian Black Sea fleet with a base in Sevastopol, and we are acting within the framework of that agreement," Russia's UN Ambassador Vitaly Churkin said on Friday.

He made the statement after a closed-door UN Security Council emergency session which was called for by Kiev's new government to discuss developments in Crimea.

"The best way to resolve the crisis is to look hard again at the February 21 agreement," Churkin said. "They need to have a constitutional dialogue and process of forming a new constitution. They need to refrain from conducting a hasty presidential election which most likely will create more friction within the country. They need to stop trying to intimidate other regions and other political forces."

Ukraine's new government, which came to power after the ousting of Russia-backed President Viktor Yanukovich, has called for fresh presidential elections on May 25.

Grappling with an enormous economic crisis, Ukraine's newly formed government's woes snowballed at signs of defiance from the Crimean government and parliament, which still sees Yanukovich as its president.

The Crimean parliament agreed that a referendum on the region's status will also take place on May 25.

Oleksander Turchinov, Ukraine's acting president, accused Russia of following a similar scenario to the one before it went to war with Georgia in 2008 over the breakaway Akhazia region which has a large ethnic Russian population.

Describing the troop deployments as Russian "provocations", he said "they are working on scenarios which are fully analogous with Abkhazia, when having initiated a military conflict, they started to annex the territory," he said in televised comments.