Seoul, South Korea CNN —  

Shortly before midnight on Thursday, a South Korean thermal observation device identified a mysterious heat spot floating down the Imjin River on the North Korean border.

The soldiers monitoring the foggy scene thought it was flotsam, perhaps a small ball. It turned out to be a defecting North Korean soldier. “The North Korean soldier came down the river with only his head above the water,” Seoul’s Joint Chiefs of Staff explained on Thursday, according to the South Korean Ministry of Defense.

Defections of soldiers over the Demilitarized Zone (DMZ) that separates the two Koreas are rare compared to crossings made by civilians over the border between North Korea and China. To cross the heavily patrolled DMZ requires getting across several fences and a dense minefield. The last time a North Korean successfully defected through the area was in December last year when two soldiers crossed the line several weeks apart.

“The figure was identified as a person about 300 meters in front of the guard post,” the South Korean military said, adding that this was the first time since 2010 that a North Korean citizen defected over the river.

The soldier was taken into custody in the central front line of the DMZ and “expressed the intent for defection.” South Korean authorities are now trying to confirm the identity of the soldier.

There had been no unusual movement from North Korean military on the front, according to the Chiefs of Staff.

There are no official statistics showing exactly how many North Koreans have fled their country, which is home to about 25 million people. South Korea says it has welcomed more than 32,000 defectors since 1998. Last year alone, the country received 1,137 defectors.

Only a handful of the defectors came through the DMZ. In November 2017, a dramatic escape of a North Korean soldier through Panmunjom village was captured on security cameras.

The video showed the North Korean soldier Oh Chong Song speeding to the border in his jeep. When the jeep got stuck in a ditch, he abandoned the vehicle to dash across amid a hail of bullets from his former fellow soldiers. He was shot five times and survived after undergoing a series of extensive surgeries in South Korea.

The North Korean military has more than 1.2 million active soldiers and a further 7.7 million in reserves. It is one of the largest ground forces in the world.