AFP reports "no signal from either one of the two captains" had been received.
Reports state 12 people have so far been rescued.
"Eight people remain missing. The search and rescue operation continues," the Maritime Rescue co-ordination Center said in a statement. "There is a threat of a gas explosion,
Russian news service TASS reports rescuers as saying the two Tanzania registered ships were 'illegally anchored'.
The incident has occurred in neutral waters outside the contested Kerch Strait, linking the Sea of Azov with the Black Sea. Russia has moved to seize the international waterway following its annexation of the adjoining Crimea Peninsula in 2014.
Russian media is reporting firefighting vessels are currently attempting to douse the flames on both ships.