A North Korean soldier has defected to South Korea, crossing one of the world's most heavily militarised borders at a time when tensions are at a peak between the neighbours.

South Korea's military confirmed the defection and said the soldier was unarmed.

The soldier, whose name has not been released yet, had walked across the demarcation line that runs through the demilitarised zone (DMZ) dividing the two rivals. He is said to have approached the South by walking across an eastern part of the front line at around 10 am local time (1 am GMT), Reuters reported.

It is reported that it is rare for defectors to opt for this route as the entire stretch of 250 km (155 miles) is always heavily guarded with barbed wires and littered with landmines. It is believed that more than 1,000 North Koreans defect to the South every year, but most of them cross over into China, and then make their way through Southeast Asia to the South.

The last time this sort of extremely rare defection through the DMZ happened was in June 2015, and before that in 2012.

South Korea's Office of Joint Chiefs of Staff said in a statement that the soldier was being held in custody for questioning to determine how and why he made the crossing.

The defection has come a day after an 18-year-old North Korean maths prodigy was suspected to have defected to Seoul. The teenager said he spent two months at the South Korean consulate in Hong Kong before arriving in Seoul.

Any defection now is seen as a huge embarrassment for North Korea as it has come at a time of heightened tensions in the Korean peninsula following Pyongyang's repeated nuclear and missile tests.

South Korea DMZ
South Korean soldiers stand guard at a checkpoint on the Grand Unification Bridge which leads to the truce village Panmunjom, just south of the demilitarised zone separating the two Koreas, in Paju, South Korea Reuters/Kim Hong-Ji

One of the most recent and prominent defectors was the London-based diplomat Thae Yong Ho. He became the highest-ranking official to defect to Seoul. Following this, Kim Jong-un announced in August that surveillance would be stepped up along the China border to stop defections.