The South Korean navy has fired warning shots at North Korean fishing boats that crossed disputed borders in the Yellow Sea, reports say.

No shots hit the vessels which retreated, said an Associated Press report citing an unnamed official with South Korea's Joint Chiefs of Staff.

The official also said that no North Korean navy ships were involved.

Skirmishes are common in the area claimed by both sides on the Korean peninsula.

A South Korean defence ministry spokesman told AFP news that there were six North Korean boats.

"Our naval patrol boats twice fired machine gun rounds at the fishing vessels. Afterwards the North Korean vessels retreated to their territory," the unnamed official said.

Reports say this was the latest in a recent series of incidents involving North Korean fishing vessels entering South Korean waters across the disputed border.

A senior South Korean military official was quoted as saying that the military will ''promptly and sternly respond, without hesitation'' if the boats continue to cross the border, in a Yonhap news agency report.