SEOUL: North Korea vowed Friday to take "merciless" military action to protect its Yellow Sea border with South Korea and warned that Seoul would pay dearly for a naval clash this week.

"Our side reminds your side again that there exists only a demarcation line in the West Sea (Yellow Sea) set by our side, and from this moment on we will take merciless military measures to protect it," its military said in a message to the South's forces.

The North refuses to recognise the sea borderline set by the United Nations after the 1950-53 war and demands that it be drawn further to the south.

The fresh warning, which prompted the United States to urge North Korea to abandon its "bellicose rhetoric," came just five days before President Barack Obama visits Seoul for talks expected to focus on Pyongyang's nuclear programme.

The letter, quoted by Pyongyang's Korean Central News Agency, renewed demands for an apology for Tuesday's clash, in which Seoul says a North Korean patrol boat was badly damaged. It 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," it said.

Seoul says the North's 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 intruder ablaze.

The North says Seoul's ships opened fire while its craft was north of the border, the scene of bloody battles in 1999 and 2002.

The letter repeated claims that South Korea's navy had engineered the clash. "The South mobilised several ships and fired thousands of shots for the rampage in the West Sea," it said.

"It was a pre-planned conspiracy by the South's right-wing and military warmongers, aimed at blocking incipient moves towards reconciliation on the Korean peninsula with a third clash in the West Sea."

A Seoul defence official has said four South Korean patrol boats fired about 5,000 rounds in total from their automated weaponry, 100 times more than the North Korean boat did.

Military sources told local media that one North Korean sailor was killed and three wounded. No South Koreans were hurt.

South Korea has sent a destroyer to reinforce the border area, and two extra patrol boats, but has said it does not want the clash to damage relations.

After months of frosty ties, the North has recently put out peace feelers to Seoul and Washington.

"After analysing the North's message our ministry saw it as the usual rhetoric," a defence ministry spokesman told AFP.

The United States Friday called for calm.

"We would (urge North Korea) to refrain... from that kind of bellicose rhetoric and, in general, avoid any kind of provocative actions that would further inflame the tension in the region," said State Department spokesman Ian Kelly.

Analysts said they do not expect immediate retaliation given the US agreement to send its special envoy on North Korea for talks in Pyongyang later this year.

"This is a typical soundbite from the North but it's unlikely for it to take any violent measures in the foreseeable future," Professor Yang Moo-Jin of the University of North Korean Studies told AFP.

The US envoy, Stephen Bosworth, will try to persuade the North to return to six-nation nuclear disarmament talks.

The North quit the talks in April and later staged a second nuclear test along with a series of missile test-launches. The United Nations tightened sanctions in response.

Leader Kim Jong-Il last month said his country is ready to return to the six-party talks if it first holds discussions with the United States to improve relations.