A North Korean soldier defected to South Korea on Thursday after crossing the border bisecting the two Koreas, Seoul's military said.

"He crossed the military demarcation line in the middle eastern sector at around 10 a.m. and defected to the South," Seoul's Joint Chiefs of Staff said in a statement.

Questioning is under way to find out what caused him to flee to the South and how he made it over, it said.

The military reportedly plans to hold follow-up questioning in collaboration with the National Intelligence Service and other related agencies, the JCS said. 

It said the North Korean soldier didn't carry any weapon with him, and there was no exchange of fire between the two Koreas during his defection, according to the defense ministry. 

"There has not been any unusual move by the North after the soldier fled North Korea. The military has put soldiers on high alert against any provocative moves by the North," a ministry official said.

Some 60 soldiers and civilians have crossed the border to defect to the South since 2010. Previously, a North Korean soldier defected to the South in June last year. (Yonhap)