SEOUL, South Korea (AP) — South Korean soldiers on Tuesday fired warning shots to repel North Korean soldiers who temporarily crossed the rivals’ land border for the second time this month, South Korea’s military said.

South Korea’s Joint Chiefs of Staff said around 20 to 30 North Korean soldiers, while engaging in unspecified construction work on the northern side of the border, briefly crossed the military demarcation line that bisects the countries as of 8:30 a.m. It said the North Korean soldiers retreated after the South broadcasts warnings and fired warning shots and the South’s military didn’t spot any suspicious activities after that.

The South also fired warning shots on June 11 after another group of North Korean soldiers briefly crossed the MDL. The Joint Chiefs of Staff said that Tuesday’s incident occurred in a different area along the central frontline region. It said it doesn’t believe the North Korean soldiers intruded the border intentionally and that the North did not return fire.

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

The border intrusions come as tensions rise between the war-divided rivals, who in recent weeks have engaged in Cold War-style psychological warfare and made it clear they are no longer bound by their landmark military agreement in 2018 to reduce tensions.