* Search goes on for 9 crew still missing
* Second vessel in difficulty in storm
* Bosphorus Strait remains open (Adds three crew rescued)
By Ece Toksabay
ISTANBUL, Dec 4 (Reuters) - Three crew members were rescued and nine still missing after their ship sank in a storm off Istanbul's Black Sea coast on Tuesday, Turkey's coastal safety authority said.
The Volgo Balt 199, which was carrying coal and had 11 Ukrainian and one Russian crew on board, sent an SOS signal at 0730 GMT off the town of Sile on Istanbul's provincial coast but all contact was then lost, shipping agent GAC Turkey said.
"Rescue operations are ongoing," the Directorate General of Coastal Safety said in a statement.
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 high 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.
Bad weather frequently forces 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.
(Writing by Nick Tattersall; Editing by Pravin Char and Stephen Nisbet)