SIMFEROPOL, Ukraine (AFP) - Fears of an assault by Russian forces on Ukrainian military bases surrounded in Crimea did not materialise overnight, a Ukrainian defence ministry spokesman said on Tuesday.

"The night was quiet," Mr Vladyslav Seleznyov, the defence ministry spokesman for Crimea, told AFP in the regional capital Simferopol.

Ukrainian officials said on Monday that Russia had given Ukrainian soldiers in Crimea an ultimatum to surrender or face an all-out assault, although Russia denounced the claim as "complete nonsense".

"No more declarations have been made by the Russian side," Mr Seleznyov said.

Russian forces have surrounded Ukrainian military bases across Crimea as the Russian-speaking autonomous region has been thrown into turmoil following the ouster last month of Moscow-backed president Viktor Yanukovych.

The Ukrainian defence ministry says some 6,000 Russian troops have been sent by the Kremlin to Crimea.

Armed men believed to be under Moscow's orders last week seized key government buildings in Crimea and airports on the peninsula.

Regional lawmakers deposed the Kiev-supporting prime minister and have called for a referendum on March 30 on proclaiming greater autonomy for Crimea.