SEOUL - The South Korean military detained a North Korean man who crossed the heavily fortified demilitarized zone (DMZ) that separates the two countries, South Korea's Joint Chiefs of Staff (JSC) said on Thursday.

The unidentified man was detected moving south near the Imjin River late on Wednesday and South Korean troops took him into custody just before midnight, the JCS said in a statement.

South Korean authorities are investigating how the man crossed the border and whether he wished to defect, the statement said.

South Korea returned three North Koreans on Monday who had crossed the maritime border aboard a fishing vessel.

Officials from the South said they thought the fishermen may have wanted to defect but in the end the men chose to return to North Korea.

Hundreds of North Koreans make their way to South Korea each year hoping to defect, but unauthorized crossings of the DMZ are rare.

The zone is dotted with landmines, fences, guard posts, and other military equipment on both sides of the border.

In November 2017, a North Korean soldier was shot multiple times by fellow soldiers as he fled across the border into South Korea, where he was treated by doctors.

Another North Korean soldier crossed the border to defect a few weeks later in an incident that led to South Korean guards firing warning shots into North Korea.

North and South Korea agreed last year to try to reduce tensions along their border by reducing the number of landmines and guard posts, disarming their guards at a truce village, and imposing a no-fly zone.