(Updates with two rescued)

SOFIA, Sept 28 (Reuters) - A Belgian yacht rescued two survivors adrift on a life raft in the Black Sea on Sunday after a North Korean-registered cargo ship with 10 crew on board sank in rough waters, Bulgarian authorities said.

The two Ukrainian seamen sent a text message to a family member late on Saturday but Bulgarian rescuers failed to find them all day on Sunday, said Nikolay Apostolov head of Bulgaria's Maritime Administration Agency.

"A Belgian yacht came across their life raft in Turkish waters this evening," Apostolov said. He spoke to one of the Ukrainians on the phone who explained that their ship, the Tolstoy, split into two and sank very quickly early on Saturday.

The survivors could not say whether anyone else from the crew of nine Ukrainians and one Russian managed to get onto a life raft, Apostolov said.

The Tolstoy, which carried 2,500 tonnes of metal and was sailing from Russia to Turkey, sank some 12 miles off Cape Emine on Bulgaria's Black Sea coast. It issued no SOS call, officials said.

Seven Bulgarian ships and a helicopter have been taking part in the rescue operation on Sunday but have not found any other survivors so far. Rescuers have also not found any bodies.

Bulgaria has asked neighbouring Turkey to send ships to help rescue any survivors.

Efforts to save the crew were hampered by stormy conditions on Saturday.

The vessel was owned by a Ukrainian company and sailed under the North Korean flag, Bulgarian authorities said. (Reporting by Anna Mudeva; Editing by Janet Lawrence)

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