In the most serious border clash in three years, a North Korean navy boat with heavy-caliber weapons sank a South Korean patrol boat Saturday, killing at least four South Korean sailors and wounding another 22, the South Korean military said. At least one sailor was missing.

There was no immediate word on North Korean casualties or missing. A Northern warship was seen being towed in flames across the maritime border, which divides North and South Korea, said army Lt. Gen. Lee Sang-hee, chief operations officer for the Joint Chiefs of Staff.

The 21-minute clash in the Yellow Sea was a blow to President Kim Dae-jung's efforts to reconcile with communist North Korea, which shares a sealed, heavily fortified border with the South. The 1950-53 Korean War ended with an armistice, not a peace treaty.

It was unclear how the incident would affect prospects for a revival of long-suspended dialogue between North Korea and the United States, South Korea's chief ally.

On Thursday, a U.S. State Department official proposed to North Korean diplomats at the United Nations that talks resume in the second week of July in Pyongyang, the North Korean capital

A North Korea expert at the Sejong Institute, a private research center in Seoul, speculated that the North might apologize secretly to the South for what he believed was an "isolated" case.

"If the case is not resolved smoothly, it will have a devastating impact on South-North relations as well as U.S.-North Korean relations," said the expert, Paik Hak-soon.

South Korea blamed North Korea for the naval clash, saying two communist boats had intruded up to 3 miles into South Korean waters. They ignored warnings to withdraw, broadcast from South Korean military boats with loudspeakers, Lee said.

"The North Korean provocation is a clear violation of the armistice agreement and we hereby strongly warn that all responsibility for this rests with the North Korean side," Lee said.

President Kim convened an emergency National Security Council meeting Saturday afternoon, and canceled plans to watch on television the South Korean soccer team's evening playoff game against Turkey in the World Cup.

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

Huge crowds were gathering in the streets of major cities to cheer their national soccer team.

All South Korean television stations, which have reported heavily on the World Cup, cut off regular programming to report on the naval battle.

The clash occurred at 10:25 a.m. when two South Korean navy vessels tried to repel two North Korean navy warships and an unspecified number of Northern fishing boats.

A South Korean navy speedboat with 27 sailors aboard took a direct hit in its steering room and caught fire. The South Korean military said the communist boats started firing first from about 500 yards away.

About 150 South Korean fishing boats operating in the area were immediately evacuated after the clash.

The maritime border between the two Koreas is not clearly marked. South Korea accused North Korea of making 12 brief border violations in the western sea last year.

In the summer of 1999, a series of border violations by North Korean ships touched off the first naval clash between the two Koreas since the 1950-53 Korean War. One North Korean warship sank and about 30 North Korean sailors died, according to South Korea. Several South Korean sailors were wounded.

The gun battle Saturday followed a series of border incursions by North Korean navy ships into South Korean waters in the area in recent months. Two communist vessels escorting North Korean fishing boats entered South Korean waters for 70 minutes Friday before South Korean patrol boats chased them back across the border, South Korean officials said.

The Koreas were divided in 1945. The United States keeps 37,000 troops in South Korea as a deterrent against North Korea.