North and South Korea were involved in a naval clash on Tuesday off the peninsula's west coast, military officials said, reportedly leaving a North Korean patrol boat badly damaged.
A spokesman for the Joint Chiefs of Staff confirmed the clash to AFP but said details were not immediately available.
A government source quoted by Yonhap news agency said a North Korean patrol boat crossed the disputed sea border, prompting South Korea's navy to fire warning shots.
When the boat continued sailing southwards, the South's navy opened fire at it, the source was quoted as saying. The North Korean boat fired back.
"There were no casualties on our side while the North Korean boat, half-destroyed, sailed back to the North," the source was quoted as saying.
The border in the Yellow Sea was the scene of deadly naval clashes in 1999 and 2002 and has always been a potential flashpoint.
The North's navy last month accused South Korea of sending warships across the border to stir tensions, and said the "reckless military provocations" could trigger armed clashes.
The border known as the Northern Limit Line was drawn up unilaterally by United Nations forces at the end of the Korean War in 1953. The North has never recognised it and wants it drawn further to the south.