North and South Korea have had a naval skirmish in Yellow Sea waters off their west coast, according to reports from South Korea's Yonhap news agency.
There have been no reports of casualties.
The Yonhap agency has cited a South Korean defence source as saying a North Korean patrol ship crossed the Northern Limit Line and did not cease when the South fired warning shots.
The BBC reports South Korean officials as saying the North Korean ship was seriously damaged before it sailed back into its territorial waters.
They say the South Korean ship was not damaged.
The incident comes just ahead of a visit to Asia by United States President Barack Obama.
