North Korea has declared that it no longer recognised the sea border with the South.

After an emergency meeting of its National Security Council, South Korea pledged to defend the existing border, the Northern Limit Line (NLL), "firmly and steadfastly".

"If North Korea violates the line," South Korea's office of the Joint Chiefs of Staff said in a statement, "we will consider it a provocation and will not condone it".

On Thursday, Pyongyang said the North Korean army would defend a new border by what it called "various means and methods".

The statement followed the failure of talks involving the two Koreas and the American-led United Nations Command which centred on the border issue.

The NLL was drawn by the United Nations after the end of the Korean War in 1953 and runs a few kilometres off the North Korean coast. North Korea wants it moved further south-west, the BBC Seoul Correspondent, Andrew Wood, says.

BBC Seoul Correspondent Andrew Wood: "Seoul wants to discuss the matter in the Joint Military Commission"

But experts said Pyongyang was unlikely to use military force to assert its expanded territorial claims.

They called North Korea's move a manoeuvre to bolster its bargaining hand ahead of crucial talks with the United States next week.

The US called on Pyongyang to respect the current border.

"We urge the DPRK (North Korea) to recognise the practicality of the NLL by keeping its craft north of the line," State Department spokesman Philip Reeker said.

The spokesman added that the line had been an effective means of preventing military tension between North and South Korean military forces for 46 years.

Stormy waters

In June, the South Korean navy sank at least one North Korean vessel in a gun battle that started after the North's patrol boats and fishing boats repeatedly strayed into waters claimed by the South.

Both sides blamed each other for sparking the clash after North Korean military vessels and fishing boats repeatedly strayed over the NLL.

North Korean fishermen cross the line every year to reach rich crab fishing grounds, the BBC Seoul Correspondent says. In June, they refused to withdraw even when challenged by the South's navy.

South Korean fishermen are reportedly concerned about the North's territorial claim which could prevent them from fishing in the area.

Still at war

Almost half a century after the end of the Korean War, the two Koreas remain technically at war because their conflict ended in an armed truce, not a peace agreement. The North has never recognised the NLL.

South Korea has warned ships not to approach border

The new signs of North Korean belligerence coincided with strong warnings by Japan and South Korea against Pyongyang.

Japan's Prime Minister Keizo Obuchi and his Seoul counterpart Kim Jong-Pil reiterated warnings over North Korea's suspected plans to test-fire a new Taepodong II long-range missile which could hit parts of the US.

North Korea shocked the region last year when it launched a medium-range Taepodong I missile over Japan and into the Pacific without prior warning.