Ukraine's new prime minister urged Russian President Vladimir Putin to pull back his military on Sunday in the conflict between the two countries, warning that "we are on the brink of disaster".

The comments from Arseniy Yatsenyuk came as a convoy of Russian troops rolled toward Simferopol, the capital of Ukraine's Crimea region, a day after Russian forces took over the strategic Black Sea peninsula without firing a shot.

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

So far, the new government has been powerless to react to Russian military tactics.

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, where they make up about 60 percent of the population, or elsewhere in Ukraine.

On the road from Sevastopol, the Crimean port where Russia has its key Black Sea naval base, to Simferopol, AP journalists saw on Sunday 12 military trucks carrying troops, an armored vehicle armed with a machine gun and also two ambulances.

US President Barack Obama spoke with Putin by telephone for 90 minutes on Saturday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity", the White House said.

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

NATO's North Atlantic Council, the alliance's political decision-making body, and the NATO-Ukraine Commission were to meet on Sunday. NATO Secretary-General Anders Fogh Rasmussen said the allies will "coordinate closely" on the situation in Ukraine, which he termed "grave".

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

Ukraine's acting president, Oleksandr Turchynov, announced late on Saturday 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.

On Crimea, however, Ukrainian troops have offered no resistance.

The new government came to power last week following months of protests against the now-fugitive president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia instead of the European Union.

Ukraine's population of 46 million is divided in 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. Crimea, a semi-autonomous region that Russia gave to Ukraine in the 1950s, is mainly Russian-speaking.

AP - Reuters