North Korea is believed to have almost doubled the number of landmines it has planted in the Demilitarized Zone that bisects the two Koreas this year from the previous year, South Korean military officials said Sunday.

Since April, the North has placed more than 4,000 new landmines in the DMZ, and 70 to 80 percent of them are believed to be wooden-box land mines which are hard to detect, according to the officials.

"The military is focusing on searching for landmines buried by North Korea on concerns that they can be washed away from the North during the heavy rainy season," said a military official.

In the summer, wooden-box land mines sometimes make their way south due to elevated water levels during the heavy rainy season.

The North Korean military is suspected to deliberately set mines to be washed away by the torrential rains.

The South Korean military said it is also keeping close tabs on the North's possible implantation of landmines near the DMZ.

In August last year two South Korean soldiers were seriously injured by a landmine explosion at the DMZ blamed on North Korea.

Millions of landmines are believed to have been laid on the divided peninsula during the 1950-53 Korean War, particularly in the DMZ.

The two Koreas still remain technically at war since the three-year war ended in a truce, not a peace treaty. (Yonhap)