Russia has announced 150,000 combat troops have been put on high alert near its border with Ukraine.

Measures are being taken to guarantee the safety of the Crimea-based Black Sea Fleet, Russian Defence Ministry Officials were quoted as saying by the BBC.

Jets have been placed at the Russian western border with Ukraine and are ready for action.

Ukrainian security forces have also been prepared for a possible escalation of violence after Thursday morning's incident on the Crimea autonomous region, where ethnic Russians from the "self-defense unit" opposing the new leadership in Kiev seized the parliament and government headquarters in the administrative capital Simferopol.

Kiev's new leadership referred to the events in Crimea as a "terrorist act".

On Thursday, short after the buildings were occupied, Russian agency Interfax reported that 50 Russian BTR vehicles had been spotted as heading towards Simferopol in order to ensure the safety of the Black Sea Fleet.

Interim Ukrainian President Oleksandr Turchynov declared that military commanders at the fleet's bases across Crimea should remain within the territories agreed upon between Ukraine and Russia.

"Any violation should be considered an act of military aggression", Turchynov was quoted as saying by Russian agency ITAR-TASS.

Turchynov warned Russia against intervening in Crimea and breaching Ukraine's sovereignty.