A North Korean vessel crossed over the Northern Limit Line (NLL) maritime border on the Yellow Sea Tuesday night and retreated upon South Korean warning shots.

A Joint Chiefs of Staff official in Seoul says the vessel, which is believed to be a North Korean patrol boat, crossed over the NLL some 700 meters around 9:38 p.m. Tuesday. The official said it stayed in South Korean waters for about nine minutes before retreating north after receiving warning shots.

The official says South Korea fired a 40 millimeter machine gun eight times over a minute from a 500 meter distance. He said the warning shots were preceded by three warnings over a loudspeaker.

The vessel is believed to have accidentally crossed the NLL while cracking down on illegal operations of Chinese fishing boats. But with intelligence in hand that the North Korean military has recently ordered the hijacking of a South Korean fishing boat on the Yellow Sea, authorities in Seoul are also looking into a possible provocation attempt.