North Korean fishing boats crossed the Koreas' tense western sea boundary and retreated soon after being warned Wednesday in the second such violation this year, South Korea said. No fighting erupted though the border is one the North has long refused to recognize.

A North Korean government boat briefly crossed the boundary while trying to guide the seven fishing boats back to North Korean-controlled waters, a South Korean Defense Ministry official said on condition of anonymity, citing office rules.

Later Wednesday, seven North Korean boats again violated the boundary, but they all sailed back after a warning was broadcast, the official said. It wasn't clear whether they were the same seven boats that earlier crossed.

Seoul says North Korean fishing boats last crossed in April. The disputed sea boundary is not clearly marked, and incursions by North Korean military and fishing boats are not unusual. Violence often erupts in the seafood-rich waters, and three naval clashes since 1999 have taken a few dozen lives.

After the Korean War ended nearly 60 years ago with a truce, not with a peace treaty, the U.S.-led U.N. Command divided the Yellow Sea without Pyongyang's consent. The boundary cut North Korea off from rich fishing waters, and Pyongyang has contested the line ever since.

North Korea shelled a front-line South Korean island near the boundary in November 2010, killing four people. Earlier in 2010, an explosion ripped apart a South Korean warship in the area, killing 46 sailors. Seoul said Pyongyang torpedoed the vessel. North Korea denies responsibility.