Six to seven fishing boats from the Democratic People's Republic of Korea (DPRK) crossed a tense maritime border with South Korea Wednesday, local media reported.

The boats were spotted at 11:20 a.m. local time in waters off Yeonpyeong Island, a front-line island the DPRK shelled in 2010, near the western sea border called Northern Limit Line (NLL), according to Yonhap News Agency.

Naval forces have reportedly asked the fishers to return to the North.

Pyongyang refuses to acknowledge the sea border, drawn unilaterally by the U.S.-led United Nations Command (UNC) at the end of the 1950-53 Korean war.