SEOUL, South Korea (AP) — The two Koreas briefly exchanged naval fire Tuesday along their disputed western sea border, with a North Korean ship suffering heavy damage before retreating, South Korean military officials said.

There were no South Korean casualties, the country's Joint Chiefs of Staff said in a statement, and it was not immediately clear if there were any casualties on the North Korean side.

The North's patrol boat crossed the disputed western sea border Tuesday morning, drawing warning shots from a South Korean navy vessel, the statement said. The North Korean boat then opened fire and the South's ship returned fire before the North's vessel sailed back toward its waters, the statement said.

The North Korean ship was seriously damaged in the skirmish while the South Korean ship was unscathed, a Joint Chiefs of Staff officer said. The officer spoke on condition of anonymity, citing department policy.

There was no immediate comment from North Korea on the clash.

Navies of the two Koreas fought deadly skirmishes along the western sea border in 1999 and 2002.

No South Korean sailors were killed in 1999, but six south Korean sailors were killed in 2002, according to the South Korean navy. It said exact North Korean causalities remain unclear.

The two Koreas have yet to agree on their sea border more than 50 years after the end of their 1950-53 civil war, which ended in an armistice and not a permanent peace treaty. Instead, they rely on a line that the then-commander of U.N. forces, which fought for the South, drew unilaterally at the end of the conflict.

North Korea last month accused South Korean warships of broaching its territory in waters off the west coast and warned of a clash in the zone, which is a rich crab fishing area.

The latest conflict comes amid international tensions over North Korea's nuclear weapons program and expectations that Pyongyang and the United States may soon engage in direct talks.

Meanwhile, there were no signs of tension along the heavily fortified land border separating the two Koreas. South Korea's Joint Chiefs of Staff said that were no unusual troop movements on the North Korean side of the land border.

At Panmunjom in the Demilitarized Zone separating the two Koreas, an Associated Press photographer said there was no unusual tension visible. A group of Chinese tourists was visiting on the North Korean side.

The area is where officers from North Korea hold meetings with their counterparts from South Korea, the United States and other members of the United Nations command.