South Korean troops fired warning shots Monday as a North Korean patrol approached the border inside the heavily fortified Demilitarised Zone (DMZ) that separates the rivals.

There was no exchange of fire and the North Korean patrol eventually moved away from the military demarcation line (MDL) marking the border, a defence ministry spokesman said.

The spokesman said the incident -- the latest of several -- occurred at about 9:40am (0040 GMT), when around 10 North Korean soldiers were observed moving towards the border line.

"After issuing verbal warnings, our side fired around 20 warning rounds," he said, adding that the North Korean patrol later retreated.

There have been a series of minor skirmishes, involving exchanges of fire but no casualties, along the inter-Korean land and sea borders in recent months.

Despite its name, the DMZ is probably the world's most heavily militarised border, bristling with watchtowers and landmines.

Four kilometres wide, it straddles the MDL which marks the actual frontier.

On October 19, warning shots fired by South Korean guards in a similar situation triggered a 10-minute exchange of fire in which nobody was hurt.

On October 10 the two sides traded heavy machine-gun fire after the North's military tried to shoot down some leaflet-laden balloons launched by South Korean anti-Pyongyang activists.

A few days before that, North and South Korean naval patrol boats had briefly exchanged warning fire near their disputed Yellow Sea border, which has been the site of numerous clashes in the past.

The minor skirmishes raised military tensions at a time when the two Koreas were trying to implement an agreement to resume high-level talks.

The dialogue has yet to begin, with each side accusing the other of lacking sincerity.