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

The South’s military observed increased North Korean construction activities along their heavily armed border to install suspected anti-tank barriers, reinforce roads and plant land mines. The work has gone on uninterrupted despite several explosions caused by mines that killed or injured an unspecified number of North Korean soldiers, said the South’s Joint Chiefs of Staff.

According to the Joint Chiefs of Staff, 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 activity 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 has said the border area is dense with overgrown trees and plants that may have obscured the visibility of North Korean soldiers and caused them to cross the border.

The Joint Chiefs of Staff said it suspects North Korea will expand its border construction activities, which started around April and is possibly aimed at making it harder for North Korean civilians or soldiers to defect to the South as Pyongyang’s leadership attempts to strengthen its control over its people.

“Our military is closely monitoring North Korean military activities in the frontline area while guarding against accidental situations,” the Joint Chiefs of Staff said in a statement.

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. The military demarcation line marks the border between the two Koreas within the DMZ, which is 248 kilometers (154 miles) long and 4 kilometers (2.5 miles) wide. The zone is strewn with an estimated 2 million mines and 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.

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