North Korean soldiers crossed the military demarcation line that divides the Koreas on Thursday, according to the South Korean military. The soldiers retreated to their side of the border after the South fired warning shots.

South Korea’s Joint Chiefs of Staff said Friday that “several North Korean soldiers crossed the border into our side of the Demilitarized Zone” at around 11 a.m. Thursday.

According to the JCS, the North Korean soldiers made it about 20 meters south of the border before the South Korean military responded with a broadcast and warning shots.

Thursday marks the third border incursion by the North Korean military this month. Both of the last times the North Korean soldiers crossed the border, dating June 9 and 18, they returned to the North following warning actions by the South.

The JCS believes the recent breaches are accidental, as the border signs are not clearly visible in those areas.