SEOUL, SOUTH KOREA -- South Korean troops fired warning shots to repel North Korean soldiers who briefly crossed the rivals' heavily fortified land border Tuesday for the second time this month, the South's military said. Both incursions were believed to be unintentional.

Around 20 to 30 North Korean soldiers who were doing construction work crossed the military demarcation line that serves as the border between the two countries in the Demilitarized Zone, according to South Korea's Joint Chiefs of Staff. The soldiers retreated after the South broadcasted warnings and fired warning shots, and the South's military didn't spot any suspicious activity after that, the joint chiefs said.

The border area is dense with overgrown trees and plants, and the North Korean soldiers may have not known exactly where the border was, according to the South's military, which said it doesn't believe the soldiers deliberately crossed into the South. The North did not return fire.

The intrusion comes as tensions rise between the 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.

Troops from the South also fired warning shots on June 11 after another group of North Korean soldiers briefly crossed the border. The Joint Chiefs of Staff said that Tuesday's incursion occurred in a different area along the central front-line region.

The Koreas' heavily fortified border area, 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 kilometres (154 miles) long and four kilometres (2.5 miles) wide.

The zone is strewn with an estimated two 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.

The South's military has observed increased North Korean activity along the border to install what appear to be anti-tank barriers, reinforce roads and plant land mines. The work has gone on uninterrupted despite several explosions caused by mines that killed or wounded an unspecified number of North Korean soldiers, said the South's Joint Chiefs of Staff.

The construction 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, according to the joint chiefs.

"Our military is closely monitoring North Korean military activities in the front-line area while guarding against accidental situations," the joint chiefs said in a statement.