Turkish rescuers were searching for eleven Ukrainian and one Russian crew members on Tuesday after their cargo ship sank in a storm off Istanbul's Black Sea coast, the coastal safety authority said.

The Volgo Balt 199, which was carrying coal, sent an SOS signal at about 0730 GMT off the town of Sile on Istanbul's provincial coast but all contact was subsequently lost, shipping agent GAC Turkey said.

"We confirmed that the ship sank. Work continues to rescue 12 people on board," Salih Orakci, head of the Directorate General of Coastal Safety, told Turkey's NTV television.

A helicopter and rescue boats were searching for the missing crew, while a second cargo ship with 14 crew members on board had experienced mechanical failure and was also at risk of sinking, GAC said. Television footage showed the second vessel listing heavily in swollen seas.

Sile lies about 40 km (25 miles) east of the northern end of the Bosphorus Strait, the only maritime outlet for cargo from Black Sea countries, including Russian oil and grain.

Adverse weather conditions frequently force the strait's closure in winter, but GAC said the Bosphorus remained open on Tuesday. About 10,000 vessels carrying 150 million tonnes of oil and petroleum products use the strait every year. – Reuters