North, South Korea naval patrol boats exchange fire

The incident came as hopes had been raised of a constructive reboot in strained inter-Korean relations following the surprise visit of a top-ranking North Korean delegation to the South just three days before.

The defence ministry said the South's patrol boat had initially fired a warning shot after the North Korean vessel penetrated half a nautical mile inside the South's territorial waters.

Instead of retreating immediately, the North patrol boat opened fire, so "our side fired back," a ministry spokesman said, adding there was "no damage" sustained by the South Korean vessel.

Although the incident went beyond warning shots, the spokesman indicated that neither side had made a concerted effort to hit the other.

The incident took place around 9:50 am (0050 GMT) near the South Korean border island of Yeonpyeong, and the North patrol boat retreated to its side of the border 10 minutes later.

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 US-led United Nations forces after the 1950-53 Korean War.

The Korean conflict ended in an armistice instead of a peace treaty, and technically the two Koreas are still at war.

Both sides complain of frequent maritime incursions by the other and there were limited naval clashes in 1999, 2002 and 2009.

In November 2010, North Korea shelled Yeonpyeong island, killing four South Koreans and briefly triggering concerns of a full-scale conflict.

The latest scrap followed the shock visit to South Korea on Saturday by some of North Korean leader Kim Jong-Un's closest aides.

It was led by Hwang Pyong-So, a newly elected vice chairman of the nuclear-armed North's powerful National Defence Commission who is widely seen as Kim Jong-Un's number two.

The visit resulted in an agreement to resume a high-level dialogue that had been suspended for seven months as military tensions on the divided peninsula soared.