(Bloomberg) — South Korean soldiers fired warning shots after North Korean troops crossed the border and then retreated, in the third such incident this month on the heavily armed peninsula.

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Several North Koreans working within the Demilitarized Zone buffer that separates the two countries crossed into South Korea’s territory on Thursday and retreated after being warned verbally, which was followed by the warning shots, South Korea’s Joint Chiefs of Staff said in a message sent to reporters Friday.

The incident took place hours after Russian President Vladimir Putin wrapped up his first visit to North Korea in 24 years, where he and Kim Jong Un reached a major deal through which the countries would come to each other’s aid if attacked.

Tensions along the border dividing the peninsula were on the rise even before Putin’s visit. North Korea has been sending large numbers of troops into the buffer zone area since April for activities such as planting mines, setting up anti-tank barriers and repairing roads, Yonhap News reported, citing military officials it did not identify.

Last month, 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 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 possibly involve the some 28,500 US military personnel stationed in South Korea.

The actual border known as the Military Demarcation Line sits within 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 by chest-high signs that can often be set far apart.

–With assistance from Soo-Hyang Choi and Seyoon Kim.

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