Deposed Ukranian president Viktor Yanukovych formally asked Moscow to deploy Russian troops to re-establish law and order in his country, Russia told the UN Security Council on Monday.

Russia's ambassador, addressing the third round of emergency talks on Ukraine in New York in four days, said Yanukovych sent the written request to President Vladimir Putin on Saturday.

Vitaly Churkin said "radical extremists" had seized power in Ukraine and - incited by Western powers - were threatening the lives and legitimate interests of Russians.

Quoting from Yanukovych's letter, and displaying a photocopy for all participants to see, Churkin said: "Ukraine is on the brink of a civil war. In the country there is chaos and anarchy.

"Under the influence of Western countries, there are open acts of terror and violence. People are being persecuted for language and political reasons.

"So in this regard I would call on the president of Russia, Mr Putin, asking him to use the armed forces of the Russian Federation to establish legitimacy, peace, law and order, and stability, and to defend the people of Ukraine," he read.

Russia had called Monday's emergency session of the 15-member Security Council, where Churkin told reporters he would lay out in "considerably more detail" Russia's position on Ukraine.

But his short address provoked a stinging rebuke from the British, French and US ambassadors, who said Russia was fabricating excuses to justify its flagrant violations of international law.

US Ambassador Samantha Power said Russia's claims had no basis in reality and that there was no evidence of violence against Russian or pro-Russian communities in Ukraine.

"Russian military action is not a human rights protection mission. It is a violation of international law," she said.

"Russian mobilization is a response to an imaginary threat. ... Military action cannot be justified on the basis of threats that haven't been made and aren't being carried out," Power added.

British Ambassador Mark Lyall Grant concurred, heavily criticizing Russia's "flagrant breach of international law", and its violation of Ukraine's sovereignty and territorial integrity.

"We can see absolutely no justification for these actions," he said.

On Saturday, the UN Security Council was mired in similar disagreement when Western nations urged Moscow to pull back its military reinforcements from the strategic Black Sea peninsula of Crimea, which has a majority ethnic-Russian population.

According to a diplomat on the council, Russia "was taken aback" on Saturday and wanted the follow-up session on Monday "to give its read on events, especially as things go south on the ground".