SEOUL, South Korea — South Korea’s military said Friday that it had fired warning shots the previous day to repel North Korean soldiers who temporarily crossed the rivals’ land border for the third time this month.

South Korea’s Joint Chiefs of Staff said several North Korean soldiers while engaging in unspecified construction work on the northern side of the border briefly intruded the military demarcation line that bisects the countries at around 11 a.m. Thursday.

The South Korean military in response broadcasted a warning and fired warning shots, to which the North Korean soldiers retreated. The joint chiefs didn’t immediately release more details.

South Korea’s military has said it believes the previous two border intrusions this month weren’t intentional as the North Korean soldiers have retreated after the warning shots and the North has not returned fire.

The South’s military says it has been observing increased North Korean construction activity in frontline border areas since April, such as installing suspected anti-tank barriers, reinforcing roads and planting land mines.