Two of the four North Koreans on board the fishing boat were rescued by the crew of the South Korean cargo vessel which was carrying sand from the North, said the official.

A spokesman for the South Korean unification ministry told reporters that the cause of the accident had yet to be determined.

"This is an accident, and we expect to follow appropriate procedures under international rules," the spokesman said on Tuesday.

North and South Korea often accuse each other of breaching their disputed sea border.