(Bloomberg) -- South Korean soldiers fired warning shots at what appeared to be dozens of North Korean troops who briefly crossed the border dividing the peninsula, in the second such incident in less than two weeks.

More than 20 North Korean soldiers who were on maintenance work crossed the line Tuesday and moved back north after the shots were fired, Yonhap News reported, citing South Korea’s Joint Chiefs of Staff.

The incident took place amid brisk North Korean activities near the border region, with its soldiers recently spotted laying new mines, renovating roads and establishing what appears to be anti-tank barriers along the border, according to the South Korean military. 

Tensions between the rivals have been on the rise since late last month when North Korea began sending hundreds of balloons carrying trash over the border after complaining about South Korea conducting surveillance flights. South Korea, in response, suspended a 2018 agreement with North Korea aimed at reducing military tensions.

The incident took place hours before Russian President Vladimir Putin is due to arrive in Pyongyang for his first trip to North Korea since 2000. The visit is set to stoke accusations from the US and its partners that Kim has supplied massive amounts of munitions for the Kremlin’s war on Ukraine.

A similar border incident took place earlier this month. Seoul said at that time it appeared to be a mistake, and not an intentional intrusion. 

The two Koreas position hundreds of thousands of troops and the bulk of their firepower near the border. The recent tit-for-tat reprisals raise the risks for a small incident to escalate quickly, and involve the some 28,500 US military personnel stationed in South Korea.

The actual border between the two Koreas known as the Military Demarcation Line sits between the 4-kilometer (2.5-mile) wide Demilitarized Zone buffer that divides the peninsula. While the DMZ is easy to spot with its rows of razor-wire fencing, the MDL is more difficult to identify, as it is mostly marked with chest-high signs that can often be set far apart.

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