Four South Korean navy men were killed and 18 wounded in a clash with North Korean patrol boats today off the west coast of the divided peninsula, South Korea said.
One South Korean vessel was sunk in the clash in the Yellow Sea, the defense ministry said.
South Korea has been co-hosting the World Cup soccer finals and the South Korean team is due to hold its last match in the southern city of Taegu on Saturday evening.
"Four South Korean personnel were killed, 18 were injured and one is missing," South Korean defense ministry official Lieutenant General Lee Sang-hee told a news conference.
"A North Korean patrol ship started shooting at around 10.25 (2.25 a.m. Irish time)...one of our navy vessels sank," he said.
The official said there were 27 navy personnel on the vessel that sank.
Two North Korean patrol ships briefly crossed the disputed maritime border - the so-called Northern Limit Line - 40 km (25 miles) west of Yeonpyongdo, an island off the west coast of the Korean peninsula, the ministry said in a statement.
The official said the two North Korean boats had returned to their waters after the clash.
It was the twelfth maritime intrusion by North Korean vessels this year including nine by military boats.
South and North Korea have been divided since the 1950-53 Korean War and are still technically at war because the fighting ended in a truce.
The Demilitarized Zone separating the two Koreas is one of the most heavily fortified frontiers in the world.
South Korea's national soccer team plays Turkey in a third place playoff today.