Ukraine calls up military reservists

Ukraine is calling up all military reserves and must ensure the armed forces are combat-ready as soon as possible, the country's top security official has said.

Andriy Paruby, Secretary of the Security Council which groups top security and defence chiefs, said on Sunday that an order had also been given to the Foreign Ministry to seek US and British help in guaranteeing the security of Ukraine.

The armed forces, he said, would step up security at energy facilities, the Reuters news agency reported.

The West is facing its biggest confrontation with Russia since the Cold War after Russian President Vladimir Putin declared the right to invade his neighbour.

Ethnic Russian forces have already bloodlessly seized Crimea, an isolated Black Sea peninsula where most of the population are Russian and Moscow has a naval base, and sought to disarm the small Ukrainian contingents there on Sunday.

Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and the Russian-speaking population in Crimea and elsewhere in Ukraine.

Of potentially even greater concern are eastern swathes of the country, where most of the ethnic Ukrainians speak Russian as a native language.

Those areas saw violent protests on Saturday, with pro-Moscow demonstrators hoisting flags at government buildings and calling for Russia to defend them.

Putin's declaration that he has the right to invade his neighbour - for which he quickly received the unanimous approval of his parliament - opened the prospect of war in a country of 46 million people on the ramparts of central Europe.

"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," the White House said after the two leaders spoke for 90 minutes.

Ukrainian Prime Minister Arseny Yatseniuk, leading a government that took power after Moscow ally Viktor Yanukovich fled a week ago, said Russian armed action "would be the beginning of war and the end of any relations between Ukraine and Russia".

Acting President Oleksander Turchinov ordered troops to be placed on high combat alert. Foreign Minister Andriy Deshchytsya said he had met European and US officials and sent a request to NATO to "examine all possibilities to protect the territorial integrity and sovereignty of Ukraine".