North and South Korean troops briefly exchanged fire across their heavily fortified land border, in the second such incident this month, the South's Yonhap news agency reported.

The exchange on Sunday happened at about 5.40pm local time [0840 GMT], after a South Korean border patrol fired warning shots at a North Korean patrol near the Military Demarcation Line dividing the peninsula, Yonhap said.

It was the second exchange of fire across the land border this month, after the two rivals traded heavy machine gun rounds on October 10.

That incident came after South Korean activists floated balloons carrying anti-Pyongyang leaflets.

There were no reports of any casualties during either exchange.

A South Korean defence ministry official said the incident occurred when a South Korean border patrol spotted North Korean troops approaching close to the Military Demarcation Line.

"Verbal warnings were issued by loudspeaker and then warning shots were fired," the official said.

"The North Koreans then opened fire on our troops who returned fire," he said, adding that the exchange lasted no longer than 10 minutes.