Multiple North Korean soldiers have been killed or injured inside the Demilitarized Zone as South Korea fired warning shots early Tuesday morning.

The South Korean military said it fired the shots to repel North Korean soldiers  who temporarily crossed the rivals’ land border for the second time this month.

An estimated 20 to 30 North Korean soldiers, while engaging in unspecified construction work on the northern side of the border, allegedly crossed the military demarcation line that bisects the countries at around 8.30am local time.

The North Korean soldiers then retreated after the South broadcasted warnings and fired shots, South Korea’s Joint Chiefs of Staff said, adding that they did not see any suspicious activity after. 

North Korean soldiers were reportedly ‘killed or injured’ by exploding landmines inside the Demilitarized Zone Tuesday morning

The South previously 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.

But they also said the North has been increasing construction activity in frontline border areas, such as installing suspected anti-tank barriers, reinforcing roads and planting land mines.

They said they recently observed several explosions suspected to have been caused by mines in areas where North Korean soldiers were deployed for construction work, but that the activities continued despite an unspecified number of injuries or deaths.

The Joint Chiefs of Staff now anticipates North Korea will expand its border construction activities, which it said could be aimed at making it harder for North Korean civilians or soldiers to escape to the South as Pyongyang´s leadership attempts to strengthen its control over its people.

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.

The Koreas´ heavily fortified border, referred to as the Demilitarized Zone, has occasionally been a site of bloodshed and violent confrontations between the rivals. 

An estimated 2 million mines are peppered inside and near 155-mile-long border, which is also guarded by barbed wire fences, tank traps and combat troops on both sides.

It´s a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty.