N. Korea Warns of Military Action Over Naval Clash

North Korea Friday threatened to take "merciless military measures" to protect its West Sea border with South Korea following a naval clash earlier this week.

"Our side reminds your side again that there exists only a sea demarcation line in the West Sea set by our side, and from this moment on we will take merciless military measures to protect it," the official news agency quoted a notice from the North's military to the South's forces as saying.

The North refuses to recognize the sea borderline set by United Nations forces after the 1950-53 Korean War and demands it be drawn further to the south.

It renewed its demand for an apology for Tuesday's clash, in which Seoul says a North Korean patrol boat was badly damaged, and called for punishment for those responsible.

"The South side will be held responsible for acts aimed at destroying national reconciliation and unity and hampering peace and unification, and it will have to pay dearly," Pyongyang's Korean Central News Agency quoted the notice as saying.

Seoul says a North Korean patrol boat crossed the border, ignored five warnings to turn back and then opened direct fire at a South Korean boat.

It said South Korean boats returned fire and set the North's boat ablaze.

North Korea says Seoul's ships opened fire while its craft was north of the border, the scene of bloody battles in 1999 and 2002. U.S. President Barack Obama is set to arrive in Seoul next Wednesday for talks expected to focus on North Korea.