A military spokesman here said North Korea has been planting some 4,000 new landmines in the DMZ since April, nearly double the previous number.

It has planted landmines in eight locations across the nominally demilitarized zone.

The North replaces landmines in the zone ahead of the rainy season every year. Most of them are box landmines that are hard to spot.

Two South Korean soldiers were severely maimed when a box landmine blew up in the DMZ in August last year. The boxes are made of wood or plastic, which makes them hard to find with metal detectors.

They are also often swept away to the South by rains because they float, making them a hazard to civilians here.

The spokesman said the military here is on alert for the North intentionally floating mines into South Korea by stealthily planting them in the South Korean-controlled part of the DMZ.

Meanwhile, the Joint Chiefs of Staff here are closely watching for possible provocations by North Korean forces on the occasion of the North's newly designated Day of the Strategic Force (July 3) and the U.S. Independence Day (July 4).

