A North Korean patrol boat retreated after briefly crossing the west sea border into South Korea on Thursday, the South's military officials said.

"A North Korean patrol boat crossed the Northern Limit Line (NLL) and sailed up to 0.36 kilometers into waters east of South Korea's Baengnyeong Island at about 11 a.m. while trying to cracking down on Chinese fishing boats illegally operating there," one of the officials said.

The Northern vessel retreated at around 11:07 a.m. after warning signs were broadcast by the South Korean military, the officials said.

About 70 Chinese fishing boats were in operation nearby and there has been no indication of unusual moves from the North, they added. (Yonhap News)