SEOUL — The South Korean military detained a North Korean soldier who crossed the heavily fortified demilitarized zone that separates the two countries, South Korea's Joint Chiefs of Staff said Thursday.

The soldier expressed his intention to defect to South Korea, the JCS said later.

The unidentified active-duty soldier was first detected by thermal imaging equipment moving south near the Imjin River late on Wednesday, and South Korean troops took him into custody just before midnight, the JCS said.

Also on Wednesday, the South Korean military recovered a body near the DMZ suspected to be that of a North Korean civilian, a spokesman for South Korea's defense ministry said Thursday.

Clad in military-type pants and a belt, the body was found in the Imjin River and appeared to have been dead for at least two weeks, South Korea's Yonhap news agency reported.

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

Officials from 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.

The last defection by a North Korean soldier across the DMZ was in November 2018.

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 ease 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.