Ukraine's Yanukovych asked for troops, Russia tells UN
Russia says ousted Ukrainian President Yanukovych asked for military aid
Ousted Ukrainian President Viktor Yanukovych asked Russia to send troops across the border to protect civilians, Moscow's UN envoy has claimed.
Vitaly Churkin told a Security Council meeting Mr Yanukovych wrote to Russian leader Vladimir Putin on Saturday.
Thousands of Russian troops have been pouring into Ukraine's Crimea region.
Moscow has argued in favour of the intervention, which has sparked outrage and threats of economic sanctions from the US and EU.
Western ambassadors dismissed Russia's arguments.
Correspondents say fears are growing that the Kremlin might try to seize more land in eastern Ukraine, where a number of people support closer ties with Moscow.
In another development, the Pentagon said is was putting on hold all military-to-military engagements between the US and Russia. These include exercises, bilateral meetings, port visits and planning conferences.
Crimea and several Russian-dominated cities in eastern Ukraine have been rocked by instability since Mr Yanukovych was ousted last month.
Monday saw pro-Russian protesters attempt to storm government buildings in the eastern city of Donetsk and Odessa, in the south.
Russian troops and pro-Moscow militias continued an apparent blockade of Ukrainian military bases across Crimea.
Kiev's UN envoy Yuriy Sergeyev says Russia has deployed about 16,000 troops to the peninsula.
And Ukrainian defence sources accused Russia's Black Sea Fleet chief Aleksander Vitko of threatening a full-scale assault if they did not surrender by dawn on Tuesday.
A Russian spokesman was later quoted as denying that any ultimatum had been issued.
But Ukrainian military personnel besieged in their bases are waiting nervously for the 03:00 GMT deadline, many of them preparing for an attack.
At the UN in New York, Mr Churkin offered the Security Council a copy of the letter Mr Yanukovych had sent.
He said the ousted leader had described Ukraine as on the brink of civil war, with civilians being persecuted simply for speaking Russian.
Vitaly Churkin (L) and Samantha Power (R) were both uncompromising in the Security Council meeting 
Ukrainian soldiers have a nervous wait as a Russian ultimatum gets closer He quoted from the letter: "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, stability and defending the people of Ukraine."
Mr Churkin reiterated Moscow's view that Mr Yanukovych is Ukraine's legitimate leader, not interim President Olexander Turchynov.
Western ambassadors refused to accept Russia's justification, continuing to accuse Moscow of violating international law.
"Russian mobilisation is a response to an imaginary threat," said US envoy Samantha Power.
"Military action cannot be justified on the basis of threats that haven't been made and aren't being carried out."
One Ukrainian navy officer said his forces were facing intense pressure from Russia to support them
UK ambassador Mark Lyall Grant also rejected Russian allegations of terrorism and threats against ethnic Russians in Ukraine.
"It is clear that these claims have simply been fabricated to justify Russian military action," he said.
He dismissed Mr Yanukovych's letter to President Putin as meaningless.
A number of Western nations have urged Russia to withdraw its troops and accept international observers in Ukraine.
The US and EU have threatened sanctions including possible travel bans and targeted economic measures.
However, a British official was photographed holding policy documents that suggest the UK will not seek to curb trade with Russia or close London's financial centre to Russians.
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