A North Korean soldier escaped to South Korea Tuesday evening by walking across the Demilitarised Zone that bisects the peninsula, Seoul's defence ministry said.

The soldier, who has not been publicly identified, approached a South Korean border guard post and expressed a wish to defect, the ministry said in a statement.

"We are holding him to investigate the motive and the process of his defection," it said without giving further details.

There was no exchange of fire during the incident, Yonhap news agency cited a military official as saying.

Over the decades since the peninsula was divided, dozens of North Korean soldiers have fled to the South through the zone which extends for two km either side of the actual border.

The most recent such case was in September last year. Prior to that, a teenage North Korean soldier defected in June 2015.

In 2012 a North Korean soldier walked unchecked through rows of electrified fencing and surveillance cameras, prompting Seoul to sack three field commanders for a security lapse.

More than 30,000 North Korean civilians have fled their homeland but it is very rare for them to cross the closely guarded inter-Korean border, which is fortified with minefields and barbed wire.

Most flee across the porous frontier with neighbouring China.

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