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 is attempting to justify the intervention, which has sparked outrage and threats of economic sanctions from the US and EU.
Western ambassadors dismissed Russia’s efforts at justification.
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 the Crimean city of Odessa.
Russian troops and pro-Moscow militias continued an apparent blockade of Ukrainian military bases across Crimea.
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.