It is unusual for North Korean naval vessels to cross into South Korean waters to nab foreign fishing boats. "A patrol boat from North Korea sailed into Korean waters about one mile off from the Northern Limit Line (NLL) in the seas off Paengnyong Island at 1:36 p.m. on Sunday,’’ the JCS said. The NLL marks the de facto maritime border between the two Koreas on the West Sea.The North Korean Navy patrol ship communicated to South Korean vessels in the area over a shared radio frequency of their intentions of grabbing the Chinese fishing boat before crossing the border, the JCS said. Warnings issued by the South Korean Navy went unheeded and the North Korean vessel returned to its own waters 25 minutes after first committing the violation, it said. "There is the possibility they crossed the border unintentionally in their efforts to crack down on the illegal fishing boats,’’ a JCS official said. Over 50 Chinese boats were reportedly fishing in the area at the time the North Korean patrol ship made the violation. Previously, North Korean patrol ships made maritime intrusions on May 13 and June 17.