A South Korean naval patrol boat is sinking in Yellow Sea; a North Korean attack is suspected.

The ship began sinking at about 9:45 a.m. eastern time after "an explosion ripped a hole" in its hull, according to the Associated Press. It was patrolling an area of the Yellow Sea that both North and South Korea claim territorial control over. Shortly after the explosion, the AP reports, the South Korean ship began firing on an unidentified target to the north.

The vessel was carrying 104 sailors, about 50 of whom have been rescued so far by South Korean navy and coast guard ships on the scene.

The two countries exchanged fire along their maritime border in November.