South Korea's navy fired warning shots Friday toward North Korean fishing boats that crossed a disputed maritime boundary, but the shots didn't hit the fishing boats and the vessels retreated, a South Korean official said.

Fishing boats routinely jostle for position in the seafood-rich Yellow Sea waters claimed by both countries during crab-catching season, which is now in high gear. Three deadly naval clashes since 1999 have taken a few dozen lives. The disputed sea boundary is not clearly marked, and incursions by North Korean military and fishing boats are not unusual.

No North Korean navy ships were involved in Friday's incident along a tense western sea boundary that the North has long refused to recognize, an official with South Korea's Joint Chiefs of Staff said, speaking anonymously because of office policy.

Six North Korean fishing boats crossed the boundary and refused to return until the South Korean Navy fired the warning shots, the official said.

North Korean fishing boats already crossed the boundary four times earlier this month but retreated each time after being warned by the South, the official said. Seoul says North Korean fishing boats also crossed the boundary in April.

The Korean War ended nearly 60 years ago with a truce, not with a peace treaty, so the U.S.-led United Nations Command divided the Yellow Sea without Pyongyang's consent. The boundary favoured South Korea, cutting North Korea off from rich fishing waters and boxing in one of its crucial deep-water ports. North Korea has bitterly contested the line ever since.

Pyongyang argues the line should run farther south. But for Seoul, accepting such a line would endanger fishing around five South Korean islands and hamper access to its port at Incheon.

In 2010, a North Korean artillery barrage on Yeonpyeong Island, which is near the boundary, killed four South Koreans, including two civilians. Also in 2010, an explosion ripped apart a South Korean warship in the area, killing 46 sailors. Seoul says Pyongyang torpedoed the vessel. North Korea denies responsibility.