An unidentified North Korean man crossed the heavily fortified land border separating the two Koreas and is in South Korean custody, the South’s military said on Friday.

The South’s Joint Chiefs of Staff said the military identified and tracked the individual near the central-west section of the military demarcation line on Thursday night. It said authorities plan to investigate the border crossing and did not immediately say whether they view the incident as a defection attempt.

The Joint Chiefs said it notified the US-led United Nations Command about the incident and had not detected any immediate signs of unusual military activity by the North.