A Russian warship carrying a marines unit has left its Black Sea port for Syria amid preparations for a possible evacuation of nationals living and working in the strife-torn country, news reports said Sunday.

The Novocherkassk landing ship is the third such craft despatched since Friday to the Tartus port that Russia leases from its last Middle East ally, agencies cited an unnamed official in the general staff as saying.

The reports said the Azov and Nikolai Filchenkov landing ships had also been sent to Syria from their Russian bases.

The military source said the Novocherkassk would arrive at Tartus within the first 10 days of January.

The Novocherkassk and another landing ship called Saratov both made a rare port call to Tartus in late November.

Officials did not disclose the details of that visit.

The Tartus base is Russia's only remaining naval station outside the former Soviet Union and is seen as a major strategic asset for Moscow.

Russia has been accused of using the base to supply Syrian President Bashar al-Assad with secret military shipments supplementing the official weapons sales that Moscow has made to Damascus since Soviet times.

But recent rebel gains prompted Russia to admit for the first time this month that Assad's days in power may be numbered.

Officials have since openly acknowledged making preparations for a possible evacuation should the safety of Russians in Syria be threatened by Assad's downfall.

The three landing ships will be joining what Russian reports said was a much broader exercise off the coast of Syria involving vessels from three naval fleets.