North Korea has recently laid tens of thousands of new landmines along its border, forcing its soldiers to work in grim conditions as they suffer multiple casualties, Seoul said Wednesday.

"Despite sweltering heat and the monsoon season, the North Korean military has laid down mines and erected wall fences along the front-line areas," the defence ministry said in a statement.

"The newly planted mines are estimated to number in the tens of thousands," it added.

During the months-long effort to strengthen defences along the demilitarized zone -- already one of the most heavily mined areas in the world -- there have been at least 10 instances where mines exploded, resulting in "multiple casualties" among the soldiers, the ministry said.

North Korean soldiers mobilised for this effort have been living under "inadequate barracks conditions without weekends or change of duty, working 12 to 13 hours a day on average", the ministry noted.

A photo released by the ministry showed three soldiers carrying what appeared to be wooden boxes, which the ministry identified as landmines.

Another photo showed a group of soldiers carrying an injured comrade on a stretcher.

South Korean soldiers are bracing for "defections" by North Korean soldiers labouring under these "poor working conditions", the ministry said, as well as further potential breaches into southern territory.

Relations between the two Koreas are at one of their lowest points in years, after the North's leader Kim Jong Un declared the South his "principal enemy" earlier this year, and moved to jettison agencies dedicated to reunification, while stepping up security along the border.

North Korean troops have crossed the border multiple times in recent months, Seoul has said, apparently accidentally during the efforts to reinforce the border.

Pyongyang has also further ramped up its weapons testing this year, as it draws ever closer to Russia. Seoul has accused Pyongyang of shipping arms to Moscow for use in the Ukraine war.