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.

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