South Korea’s military on Monday said explosions that injured three of its officers last week inside the heavily fortified Demilitarized Zone separating the Koreas were probably caused by North Korean mines.

Axar.az reports, citing teh Associated Press, the South’s Joint Chiefs of Staff announced the preliminary finding after inspecting the site with the U.S.-led United Nations Command.

The assessment could further heighten tensions between the rivals. Relations have deteriorated sharply in recent years amid North Korea’s accelerating nuclear weapons program and the collapse of U.S.-led diplomacy aimed at resolving the nuclear standoff.

Three South Korean military officers were injured in two consecutive explosions Sept. 21 while establishing a patrol route near the military demarcation line in the western section of the Demilitarized Zone.

South Korean officials initially said the blasts appeared to have been caused by mines but did not attribute them to North Korea.