The Associated Press is reporting that a South Korean naval ship was sunk near North Korean waters, and the Government of South Korea has not ruled out the possibility of an attack by its northern rival.

The official said at least 58 of the 104 crew members have been rescued. There was no immediate confirmation of casualties. A rescue operation was still under way.

The cause of the sinking was not immediately clear.

South Korea’s military scrambled naval vessels to the western waters near the disputed maritime border with rival North Korea late Friday after an explosion ripped a hole in the bottom of a military ship, officials and news reports said.