North Korea has laid tens of thousands of new landmines along its border

South Korean authorities said Wednesday that North Korea recently planted tens of thousands of new landmines along its border, a move Seoul said was endangering North Korean soldiers who were forced to work in harsh conditions.

“Despite the sweltering heat and the monsoon season, the North Korean military has been laying mines and erecting fence walls along the frontline areas,” the Defense Ministry said in a statement. “The number of newly laid mines is estimated to be in the tens of thousands,” it added.

During months of efforts to bolster defenses along the demilitarized zone, already one of the most heavily mined areas in the world, there have been at least 10 cases of mines exploding, causing “multiple casualties” among soldiers, the South Korean ministry was quoted as saying by AFP. North Korean soldiers mobilized for the effort are living in “inadequate barracks conditions, with no weekends or shift changes, working 12 to 13 hours a day on average,” the ministry noted.

A photo released by the ministry shows three soldiers carrying what appear to be wooden crates, which the ministry identified as landmines. Another photo shows a group of soldiers carrying a wounded comrade on a stretcher.

South Korean troops are preparing for “defections” of North Korean soldiers working in such “poor working conditions,” the ministry said, as well as possible further intrusions into South Korean territory.

Relations between the two Koreas are at their lowest point in years after North Korean leader Kim Jong Un declared the South his “main enemy” earlier this year and moved to scrap agencies dedicated to reunification while stepping up security along the border.

North Korean troops have crossed the border several times in recent months, Seoul has said, apparently by accident during efforts to reinforce the frontier. Pyongyang has also stepped up weapons testing this year amid a rapprochement with Russia. Seoul accuses Pyongyang of delivering weapons to Moscow for use in the war in Ukraine.