North Korea has been laying fresh landmines on its side of the Demilitarized Zone (DMZ) with South Korea, the UN Command said Tuesday, following a spate of high-profile defections.

Military personnel were seen planting mines on the North's side of a river crossing known as the Bridge of No Return -- close to the border truce village of Panmunjom, a spokesman for the UN Command, which oversees the Korean War armistice, told AFP.

In a statement the UN Command "strongly" condemned the Korean People's Army (KPA) activity.

"The presence of any device or munition on or near the bridge seriously jeopardises the safety of people on both sides," it said.

The statement added that thousands of visitors -- often school-aged children -- take part in tours to the DMZ.

Despite its name, the DMZ separating the two Koreas is one of the world's most heavily militarised frontiers, bristling with watchtowers and landmines.

It acts as a buffer zone, stretching two kilometres on either side of the actual frontier line.

Because the 1950-53 Korean War ended with an armistice rather than a formal peace treaty, the two Koreas remain technically at war.

The UN Command declined to "speculate" on why the KPA was engaged in laying fresh mines. But South Korea's Yonhap News Agency cited a military source as saying it may be an attempt to prevent front-line troops from defecting.

Cross-border Korean tensions are currently running high, with North Korea on Monday threatening nuclear strikes as South Korea and the United States began a large-scale military exercise which Pyongyang views as a provocative rehearsal for invasion.

The North has also been rocked by a series of defections, most recently that of its deputy ambassador to Britain who fled to the South in a major propaganda victory for Seoul.

Yonhap said the South Korean military was using banks of loudspeakers along the border to crow about the defection.

In August last year South Korea accused North Korea of planting landmines that maimed two soldiers on border patrol.