South Korea's coastguard said a Cambodian cargo ship has sunk after a suspected collision with an unidentified ship.
Coastguard official Lee Sung-kang said four bodies had been found and seven other sailors were missing in waters off South Korea's southeastern port of Ulsan.
He said a Russian sailor was rescued immediately after the accident on Wednesday.
Lee said the nationalities of the missing crew members from the 1,500-tonne cargo ship were not immediately available.
He added that coastguard vessels and helicopters were searching the area for them.
Coastguard officials are trying to determine the cause of the accident.