South Korean authorities said last night that a North Korean "spy boat" had been sunk after a sea chase off the southeastern coast.

They said four men -- a South Korean sailor and three fishermen -- were killed when the fleeing vessel fired on pursuing ships of the South Korean Navy.

It was the second armed confrontation between the two countries this week and threatened to cast a cloud over the current negotiations of their representatives who are attempting to set up a meeting between the prime ministers to discuss unification and other bilateral issues.

A South Korean representative to those talks said the latest shooting incident could have a "grave impact" on those talks.

The naval shootout was the first such incident since the current round of talks began early this year, largely at the initiative of the North Korean government, which had seemed eager to promote a thaw in relations with the South.

[A U.S. official in Washington said American forces in the area were able to confirm that there had been a fire fight in the area, but that there was no independent confirmation that the boat had been sunk.]

Small-scale gun battles have occurred several times in the past. On two occasions last year, the South claimed its ships had shot and sunk alleged spy boats dispatched from the North.

The absence of incidents so far this year had contributed to the relaxed atmosphere in which the preliminary talks had taken place and had been interpreted in Seoul as a sign that the communist government of President Kim Il Sung wanted to promote an easing of tension to ensure a peaceful meeting later of the two prime ministers.

Lee Don Bok, one of the South's three delegates on the talks, said the incident "has established a grave impact on the current dialogue," but did not say whether it would be discussed at the next North-South meeting scheduled for April 1.

The latest skirmish occurred early Tuesday morning about 12 miles off of the South Korean port city of Pohang, the site of a large industrial complex that includes a major steel mill and oil refinery. Pohang is about 160 miles southeast of Seoul.

South Korean officials said the boat tried to flee when it was approached by a South Korean naval vessel. It began firing, killing the South Korean seaman and three fishermen who happened to be in the vicinity. South Korean military aircraft joined the chase and the North Korean boat was sunk 35 nautical miles east of Pohang about 2 1/2 hours later, the South's authorities said. There were no survivors.

A South Korean Defense Ministry spokesman said this morning that the vessel may have been trying to land communist infiltrators in order to promote "social unrest" in the South. It may also have been collecting information or probing the South's coastal defenses, the spokesman said.

In this week's other confrontation, South Korean soldiers Sunday shot and killed three men who authorities claimed were North Korean "frogmen" attempting to infiltrate the South.

Officials said the three were killed near the Demilitarized Zone after they swam across an estuary of the Han River, about 15 miles northwest of Seoul, the capital.

The official announcement said that the intruders had been carrying Czech-made submachine guns, radios, ammunition and grenades. One of them had brought a gun fitted with a silencer to muffle the sound of shots, officials said, asserting that it had been intended for use in assassinations.

The North Koreans had appeared eager to begin the unification talks and made significant concessions to the South, agreeing to talk at government level for the first time, instead of insisting on meetings only of political parties and social groups.

North Korean spokesmen have indicated that the new, apparently more conciliatory attitude is largely due to the fact that former president Park Chung Hee of South Korea no longer is in the picture, having been the victim of an assassination on Oct. 26.

Some observers speculate, however, that the North is more willing to talk because they perceive that Seoul has been weakened by internal turmoil since Park's death.