South Korea said its navy patrol boats fired warning shots Friday at six North Korean fishing boats that crossed their disputed Yellow Sea border in the latest of a series of incursions.
None of the North Korean vessels were hit and they swiftly returned to their side of the western sea boundary after the incident, a Defence Ministry spokesman told AFP.
"Dozens of Vulcan machine gun rounds were fired into waters near North Korean fishing boats which violated the sea border," the spokesman said.
"The operation, involving two naval patrol ships, began around 3:00 pm (0600 GMT) after our side broadcast warning messages. All North Korean boats had retreated by 4:00 pm," he added.
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, followed a series of recent border violations by North Korean fishing vessels.
It was the first time for two years that the South has resorted to firing warning shots to push the fishing boats back.
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 conflict was concluded with a truce rather than a peace treaty, and small border incidents in the past have been known to escalate swiftly.
In 1999 a major battle erupted after North Korean naval ships and 20 crab-catching fishing boats crossed the line.
The South Korean navy mobilised 10 ships, some of which rammed the North Korean vessels, prompting exchanges of machine gun and artillery fire.
South Korea estimated that some 20 North Korean sailors were killed and 30 injured, while two South Korean ships were damaged and seven sailors injured.
In 2002, patrol boats from both sides exchanged fire and one South Korean vessel was sunk and six sailors killed.
Another naval clash in 2009 ended with patrol boats damaged on both sides, but only minor casualties.
The recent spate of fishing boat incursions "clearly seems to have a reason", said another military official cited by Yonhap on Friday.
The North "may try to disturb South Korea by creating military tension ahead of (December's) presidential election," the official said.
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.