A South Korean patrol boat has been sunk in a naval battle with North Korea - the most serious maritime clash between the two countries for three years.

At least four South Korean sailors were killed and more than 20 wounded in the battle, and a North Korean vessel was towed from the scene in flames, defence officials said in Seoul.

President Kim (left) 'cannot tolerate' what has happened

South Korean President Kim Dae-jung, after an emergency meeting of his National Security Council, called the incident "a flagrant violation of the cease fire agreement which we will never overlook".

Correspondents in Seoul say the battle marks the lowest point in Korean relations since a historic summit between the two sides two years ago.

South Korea has put all its armed forces in high alert.

The incident has come as a shock in South Korea, which has been in festive mood as it co-hosts football's World Cup.

The United Nations border monitoring force has called on the two sides to hold urgent talks on the incident.

"There has been no reply yet from the North Korean side," the AFP news agency reports an American military spokesman as saying.

'Repeated violations'

The 20-minute battle took place on Saturday morning, in the Yellow Sea off the west coast of the Korean peninsula.

Recent sea incidents here

1998 June - Nine dead North Korean commandos found in South Korean waters 1998 July - Dead North Korean frogman found on South Korean coast 1998 December - South Korean navy sinks North Korean spy vessel 1999 June - 30 North Korean sailors believed killed in naval battle

The incident follows a series of recent reported violations of the maritime border by North Korean vessels.

North Korea does not recognise the border, which was unilaterally imposed by the UN after the Korean War.

South Korean officials say a warning signal was given after two North Korean patrol vessels escorting fishing boats crossed the Northern Limit Line.

They say a North Korean vessel then started shooting and South Korean navy ships returned fire.

One South Korean sailor is still missing.

North Korea says there were "losses", but has given no details.

More than 100 South Korean fishing boats operating in the area were evacuated.

The disputed waters are rich in crabs.

Military 'prepared'

North Korea has accused the south of a "grave provocation" by firing on its patrol boats which it said were on "routine coastal guard duty".

But earlier, Lieutenant General Lee Sang-hee of the South Korean Defence Ministry told journalists: "This provocative behaviour by North Korea is a blatant armistice violation for which North Korea bears full responsibility."

As President Kim Dae-jung assesses what measures South Korea will take, his spokesman has called for calm.

"We urge the people to continue their everyday life with ease and resolution as the military and the government is thoroughly prepared," Park Sun-sook said

President Kim has abandoned plans to watch South Korea's World Cup play-off match with Turkey as he meets his security officials.

On Friday, South Korean officials said that two Northern patrol boats had briefly crossed the Northern limit line before returning to the North.

They said another patrol boat entered South Korean waters on Thursday.

Three years ago a series of border violations by North Korean ships sparked the first naval clash between the two Koreas since the end of the Korean War in 1953.

One North Korean warship sank and about 30 sailors were believed to have died.