A teenage North Korean soldier has walked across the world's most heavily militarised border in a bid to defect to South Korea, South Korean defence ministry officials said.

The soldier approached a remote South Korean guard post in Gangwon province's Hwacheon county, in the central area of the peninsula, at about 8am local time on Monday, a defence ministry official said. 

The soldier was being held in custody while officials ran checks.

While there are more than 1,000 defections from North Korea to South Korea every year, most defectors come via China and it is rare for a North Korean to cross the heavily mined Demilitarised Zone (DMZ).

Women peace activists cross North-South Korea border

The last such crossing was in 2012.

Monday's defection sparked a tense standoff between North and South Korean border guards across the 4km-wide and 248km-long DMZ, but there was no conflict, the Yonhap news agency reported.

The DMZ is marked by barbed wire and guarded by tens of thousands of troops on both sides.

Most North Koreans who flee repression and poverty at home cross the porous frontier with China first before travelling through a Southeast Asian nation and eventually arriving in South Korea.

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.

In August last year, two North Koreans swam across the Yellow Sea border to a South Korean front-line island.

Source: Agencies