The US has condemned North Korea's "armed provocation" against South Korea after a naval clash that killed four South Korean sailors and left at least 20 wounded.

"We support the stance of our ally against armed provocation," State Department spokeswoman Brenda Greenberg said.

South Korean officials said North Korean vessels crossed the disputed border between the two countries on Saturday and fired at its boats, triggering a gun battle.

North Korea said that in fact it was the South which committed a "grave provocation" by opening fire on the patrol boats, which it said were on "routine coastal guard duty".

Expressing its concern over the incident, China called on the two governments to safeguard stability on the Korean peninsula.

Unknown losses

A South Korean patrol boat was sunk in the incident, which took place in the Yellow Sea, and a North Korean vessel was also towed away from the scene in flames.

North Korea has admitted to suffering "losses", but given no details.

Forces on both sides are on high alert

The battle was the most serious maritime clash between the two countries for three years and a setback to recent reconciliation efforts on the divided peninsula.

The armed forces in both countries are now on high alert and the White House says it is monitoring the situation.

"Naval skirmishes have happened in the past. They are always a matter of concern, and we're monitoring the situation," said White House spokesman Ari Fleischer.

Repercussions

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

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 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.

US General Leon LaPorte, who commands 37,000 American troops stationed in South Korea as a deterrent against North Korea, warned that the attack could have serious repercussions.

"This provocative act by North Korea is a serious violation of the Armistice Agreement and could have serious implications in many areas," he said.

Border dispute

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

The incident has shocked South Korea which was celebrating the World Cup

North Korea does not recognise the border, which was unilaterally imposed by the UN after the Korean War ended in 1953.

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.