Two Russian warships crossed Turkey's Bosphorus Strait and headed towards the Black Sea en route to Ukraine, which is engaged in a military stand-off with Moscow, media reported on Tuesday.

Turkey's state-run Anatolia Agency reported that the ships, the 150 Saratov and 156 Yamal, were recalled after Russia stepped up efforts to strengthen its military presence in Crimea, Ukraine's mainly Russian-speaking autonomous region at the epicentre of the escalating crisis.

Crimea's port of Sevastopol has been the home of the Russian navy's Black Sea fleet for some 250 years and a bastion of pro-Kremlin sentiment since the collapse of the Soviet Union in 1991 saw it cut off from Moscow.