A Hyundai-built container ship collided with the freighter Wednesday while it was undergoing tests off the shipyard in the southeastern port of Ulsan.

"The cause of the accident is being investigated by authorities, but we first of all express sincere regret at human losses," Hyundai Heavy Industries, the world's largest shipbuilder, said in a statement.

The 1,500-ton Alexandra with 12 Russian crew on board was torn apart and sank off Ulsan. Coastguard ships rescued one crew member and recovered four bodies.

The coastguard said seven others are still missing despite an overnight search involving six ships and two helicopters.

Yonhap news agency said the South Korean captain of the 88,000-ton container ship was held by coastguard officials for questioning.