Ukraine's new prime minister has urged Russian president Vladimir Putin to pull back his troops in the dispute between their two countries over Crimea, saying "we are on the brink of disaster".

Arseniy Yatsenyuk spoke after a closed session of his new parliament in Kiev, calling on Mr Putin to "pull back his military".

A convoy of hundreds of Russian troops headed toward the capital of Ukraine's Crimea region today, a day after Russia's forces took over the strategic Black Sea peninsula without firing a shot.

It was reported that hundreds of unidentified gunmen arrived outside Ukraine's infantry base in Privolnoye in its Crimea region.

The convoy included at least 13 troop vehicles each containing 30 soldiers and four armoured vehicles with mounted machine guns. The vehicles - which have Russian licence plates - have surrounded the base and are blocking Ukrainian soldiers from entering or leaving it.

Ukrainian soldiers, with clips in their weapons, have positioned a tank at the gate.

"There was no reason for the Russian Federation to invade Ukraine," Mr Yatsenyuk said after the closed session of his new parliament.

So far, the new government in Kiev has been powerless to react to Russian military tactics. Armed men in uniforms without insignia have moved freely about the key peninsula, occupying airports, smashing equipment at an air base and besieging a Ukrainian infantry base.

Russia has its key Black Sea Fleet stationed on the Crimean peninsula - which was part of Russia until 1954 - and nearly 60% of Crimea's residents identify themselves as Russian.

Mr Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and Russian speakers in Crimea and elsewhere in Ukraine. However, there has been no sign of ethnic Russians facing attacks in Crimea or elsewhere in Ukraine.

President Barack Obama spoke with Mr Putin by telephone for 90 minutes yesterday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said. Mr Obama warned that Russia's "continued violation of international law will lead to greater political and economic isolation".

The new Ukrainian government came to power last week following months of pro-democracy protests against a pro-Russian president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia instead of the European Union. Yanukovych fled to Russia after more than 80 people died, most of them demonstrators killed by police. He insists he is still president.

Since then, tensions have risen sharply between the two capitals.

Ukraine's acting president, Oleksandr Turchynov, announced yesterday that he had ordered Ukraine's armed forces to be at full readiness because of the threat of "potential aggression." He also said he had ordered stepped-up security at nuclear power plants, airports and other strategic infrastructure.

Ukraine's population of 46 million has divided loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the EU, while eastern and southern regions look to Russia for support.

The Interfax news agency reported the speaker of Crimea's legislature, Vladimir Konstantinov, as saying the local authorities did not recognise the government in Kiev. He said a planned referendum on March 30 would ask voters about the region's future status.

But the US and other Western governments have few options to counter Russia's military moves.

Nato's secretary general said Russia had violated the UN charter with its military action in Ukraine, and he urged Moscow to "'de-escalate the tensions." Nato chief Anders Fogh Rasmussen spoke in Brussels as he was about to open a meeting of the alliance's political decision-making body to discuss the crisis.

Ukraine is not a Nato member, meaning the US and Europe are not obliged to come to its defence. But Ukraine has taken part in some alliance military exercises and contributed troops to its response force.

The US also said it will suspend participation in preparatory meetings for the Group of Eight economic summit planned in June to be held at Russia's Black Sea resort of Sochi, site of the 2014 Winter Olympics.

French foreign minister Laurent Fabius agreed, saying on Europe 1 radio that planning for the summit should be put on hold.

France "condemns the Russian military escalation" in Ukraine, and Moscow must "realise that decisions have costs," he said.