The South Korean military reported that South Korean troops fired warning shot to repel North Koreans who briefly crossed their heavily fortified border on Tuesday, for the second time in a month. Both incursions are believed to have been unintentional.

According to the Joint Chiefs of Staff of South Korea, around 20 to 30 North Korean construction workers crossed the military demarcation lines that serve as the border between both countries in the Demilitarized zone. Joint Chiefs of Staff said that the soldiers fled after the South fired warning shots and broadcasted warnings. The South’s military did not detect any suspicious activity following this.

According to the South Korean military, the area along the border is thickly overgrown with trees and plants. The soldiers could not have known where the border was. The North did no return fire.

The intrusion occurs as tensions between the rivals are rising. In recent weeks, they have engaged in Cold War style psychological warfare to make it clear that they are not bound by their landmark agreement on military cooperation in 2018.

On June 11, troops from the South fired warning shots after another group North Korean soldiers briefly crossed over the border. The Joint Chiefs of Staff stated that the Tuesday incursion took place in a different part of the central frontline region.

Demilitarized zone, the heavily fortified border between Koreas, has been the scene of violent clashes and bloodshed. The military demarcation lines marks the border between two Koreas in the DMZ. It is 248 km (154 miles), long and 4 km (2.5 miles).

It is guarded on both sides by combat troops, barbed-wire fences and tank traps. This is a legacy from the 1950-53 Korean War which ended in an armistice and not a peace agreement.

South Korean military officials have observed an increase in North Korean activity near the border, including increased efforts to reinforce roads, install landmines and install anti-tank barriers. The South’s Joint Chiefs of Staff said that the work continued despite several explosions from mines which killed or injured an unspecified amount of North Korean soldiers.

According to the Joint Chiefs, the construction began in April. It is likely to be aimed at preventing North Korean soldiers or civilians from defecting to the South. Pyongyang wants to tighten its grip on its citizens.

The Joint Chiefs of Staff said that “Our military closely monitors North Korean military activity in the frontline area, while guarding against any accidental situations.”