SEOUL, South Korea (AFP) — Three South Korean military officers were injured in an explosion during an operation near the heavily mined border with North Korea, Seoul’s military said Monday.

The incident occurred south of the Military Demarcation Line inside the Demilitarized Zone (DMZ), one of the world’s most heavily fortified and mined frontiers.

Two of the injured officers were evacuated by helicopter, while a third was due to be transported by road before being airlifted, the Joint Chiefs of Staff (JCS) said.

It was not immediately clear whether the explosion was caused by a North Korean mine, a South Korean mine or another explosive device.

The three officers “were injured by an explosion of unknown cause while conducting an operation at a location south of the MDL,” the JCS said in a statement.

The military did not immediately provide details on the severity of their injuries.

The DMZ is a 248-kilometer-long and roughly four-km wide buffer zone that has divided the Korean Peninsula since the 1953 armistice that halted the Korean War.