South Korea’s military says it fired warning shots when North Korean soldiers approached the Military Demarcation Line between the two countries Thursday.

South Korea’s Joint Chiefs of Staff(JCS) say the incident happened about an hour and a half after a North Korean soldier had defected across the tense border at 8:04 a.m. local time.

Military officials say the pursuing North Korean party approached the border at around 9:24 a.m., after which South Korean soldiers issued warning messages and fired about 20 warning shots at 9:30.

The Joint Chiefs say the North’s soldiers then ceased their approach and turned back.