South Korea’s military concluded that North Korea planted the mines that exploded near the border last month, injuring three South Korean soldiers.

The military said its conclusion was based on a joint investigation with the U.N. Command, forensic testing and the discovery of another North Korean mine near the blast site.

South Korea called on North Korea to apologize and remove other mines, saying the mine placements violated the armistice agreement that ended the Korean War.

Tensions remain high along the heavily fortified border, with South Korea reporting numerous instances of North Korean soldiers crossing the military demarcation line during recent fortification work, NBC News has reported.

The incident comes as South Korea seeks to revive talks with North Korea and the United States, while North Korea continues to reject dialogue and has threatened retaliation over disputes along the border.