A “low-ranking” North Korean soldier escaped to South Korea through the Demilitarized Zone between the two countries, which caused both North and South Korean troops to fire shots.

North Korean border guards surrounded the military demarcation line and appeared to search for the soldier after he defected shortly after 8 a.m. local time, according to Yonhap News Agency. This prompted South Korean troops to fire approximately 20 warning shots.

The soldier remains in custody as authorities learn why he defected, according to a South Korean Ministry of Defense official.

Shots were heard from the North Korean side about 40 minutes after the defection.

This is the fourth soldier to defect from North Korea to South Korea in 2017.

Just last month, a North Korean soldier defected through the DMZ. The soldier was headed for the border of South Korea in an automobile when a wheel from his vehicle came loose, causing him to flee from North Korean soldiers’ fire on foot on Nov. 13.

Approximately 40 rounds were shot at him, Suh Uk, a South Korean military official, told Reuters.

The North Korean military is one of the largest worldwide, with more than 1.2 million active soldiers.