Dozens of armed men in Russian-marked military uniforms are said to have occupied an airport in Ukraine's key Crimea region today - then left after finding no Ukrainian troops had landed.

Witnesses told the Interfax news agency that the 50 or so men were wearing the same gear as the ones who seized government buildings in the city, Simferopol, early yesterday and raised the Russian flag.

The report said the men with "Russian Navy ensigns" first surrounded Simferopol Airport's domestic flights terminal but a later report, datelined Moscow, quoted an airport representative as saying the men apologised and left and that the airport was operating normally.

A woman who answered the phone at the airport said "no comment" and the airport's website listed the morning's first flight, to Moscow, as boarding on schedule.

The events in the Crimea region have heightened tensions with neighbouring Russia, which scrambled fighter jets to patrol borders in the first stirrings of a potentially dangerous confrontation reminiscent of Cold War brinksmanship.

Russia also has granted shelter to Ukraine's fugitive president, Viktor Yanukovych, after recent deadly protests in Kiev swept in a new government.

While the government in Kiev, led by a pro-Western technocrat, pledged to prevent any national break-up, there were mixed signals in Moscow. Russia pledged to respect Ukraine's territorial integrity.

Mr Yanukovych was said to be in a luxury government retreat, with a news conference scheduled Friday near the Ukrainian border. He has not been seen publicly since Saturday.

Yesterday, as masked gunmen wearing unmarked camouflage uniforms erected a sign reading "Crimea is Russia" in Simferopol, Ukraine's interim prime minister declared the Black Sea territory "has been and will be a part of Ukraine".

The escalating conflict sent Ukraine's finances plummeting further, prompting Western leaders to prepare an emergency funding package.

Mr Yanukovych, whose abandonment of closer ties to Europe in favour of a bail-out loan from Russia set off three months of protests, finally fled by helicopter last week as his allies deserted him.

The humiliating exit was a severe blow to Russian president Vladimir Putin, who had been celebrating his signature winter Olympics even as Ukraine's drama came to a crisis. The Russian leader has long dreamed of pulling Ukraine - a country of 46 million people considered the cradle of Russian civilisation - closer into Moscow's orbit.

For Ukraine's neighbours, the spectre of a break-up evoked memories of centuries of bloody conflict.

"Regional conflicts begin this way," said Polish foreign minister Radoslaw Sikorski, calling the confrontation "a very dangerous game".

Russia's dispatch of fighter jets yesterday to patrol borders and drills by 150,000 troops - almost the entirety of its force in the western part of the country - signalled strong determination not to lose Ukraine to the West.

The dramatic developments posed an immediate challenge to Ukraine's new authorities as they named an interim government for the country, whose population is divided in loyalties between Russia and the West.

Crimea, which was seized by Russian forces in the 18th century under Catherine the Great, was once the crown jewel in Russian and then Soviet empires.

It only became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia - a move that was a mere formality until the 1991 Soviet collapse meant Crimea landed in an independent Ukraine.

In the capital Kiev, the new prime minister said Ukraine's future lay in the European Union, but with friendly Russian relations.

Arseniy Yatsenyuk, 39, now faces the difficult task of restoring stability in a country that is not only deeply divided politically but on the verge of financial collapse.

He served as economy minister, foreign minister and parliamentary speaker before Mr Yanukovych took office in 2010 and is widely viewed as a technocratic reformer who enjoys the support of the US.

Shortly before MPs chose him, Mr Yatsenyuk insisted the country would not accept the secession of Crimea. The Black Sea territory, he declared, "has been and will be a part of Ukraine".

But in Simferopol, tensions soared when gunmen toting rocket-propelled grenades and sniper rifles raised the Russian flag over the region's parliament building. They wore black and orange ribbons, a Second World War Russian symbol of victory.

A pro-Russian activist who gave only his first name, Maxim, said he and others were camped overnight outside the parliament when about 50 men wearing flak jackets and carrying rocket-propelled grenade launchers and sniper rifles took over the building.

"They were asking who we were. When we said we stand for the Russian language and Russia, they said, 'Don't be afraid. We're with you'. Then they began to storm the building, bringing down the doors.

"They didn't look like volunteers or amateurs; they were professionals. This was clearly a well-organised operation."

Oleksandr Turchynov, who stepped in as acting president after Mr Yanukovych's flight, condemned yesterday's assault as a "crime against the government of Ukraine" and warned that any move by Russian troops off of their base in Crimea "will be considered a military aggression".

In a bid to shore up Ukraine's fledgling administration, the International Monetary Fund said it was "ready to respond" to Ukraine's bid for financial assistance. The European Union is also considering emergency loans for a country that is the chief conduit of Russian natural gas to western Europe.

Western leaders lined up to support the new Ukrainian leadership, with the German and British leaders warning Russia not to interfere.

"Every country should respect the territorial integrity and sovereignty of the Ukraine," Prime Minister David Cameron said after a meeting with German chancellor Angela Merkel in London.

Nato defence ministers met in Brussels, and US defence secretary Chuck Hagel emerged, appealing for calm.

"These are difficult times," he said, "but these are times for cool, wise leadership on Russia's side and everyone's side."

An Associated Press photographer saw men with assault rifles patrolling the airport.

It was not immediately clear who the men were. They refused to talk to journalists.