“The operation, involving two naval patrol ships, began around 3pm after our side broadcast warning messages. All North Korean boats had retreated by 4pm,” he said.

There was no immediate comment from the North Korean side.

The incident, which occurred close to Yeonpyeong island on the South side of the border, was the latest in a series of incursions by North Korean fishing vessels in recent weeks.

Earlier Friday, Yonhap had quoted an unidentified senior military official as saying the navy would take action if the incursions continued.

“If North Korean boats repeatedly cross (the border) for fishing, the military will promptly and sternly respond, without hesitation,” the official said.

The de-facto maritime boundary between the two Koreas – the Northern Limit Line – is not recognised by Pyongyang, which argues it was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean war.

The two Koreas remain technically at war since the Korean war was concluded with a truce rather than a peace treaty, and small border incidents in the past have been known to escalate swiftly.

The maritime boundary was the scene of deadly naval clashes in 1999, 2002 and 2009.

Cross-border tensions have been especially high since the South accused the North of torpedoing one of its warships with the loss of 46 lives in March 2010.

The North angrily denied involvement but went on to shell Yeonpyeong island in November of the same year. The attack killed four South Koreans and briefly sparked fears of a full-scale conflict.

The South subsequently strengthened manpower and weaponry on its frontline islands to forestall any fresh assault.