SEOUL, May 27 (Xinhua) -- A patrol boat and a fishing boat from the Democratic People's Republic of Korea (DPRK) returned to the north after crossing the maritime border with South Korea as the South Korean military fired warning shots.

South Korea's Joint Chiefs of Staff (JCS) said Friday that the DPRK patrol and fishing boats violated some 640 meters south of the Northern Limit Line (NLL) in the west sea at about 7:30 a.m. local time.

The South Korean Navy fired five warning shots with 40-mm warship guns after issuing warning broadcasts, and then the DPRK boats returned to the north at 7:38 a.m.

It marked the second time this year since Feb. 8 that DPRK vessels have crossed the NLL illegally.