(Bloomberg) — South Korea’s military said it fired warning shots after several North Korean soldiers briefly crossed into its territory and retreated Sunday, in a move that could stoke tensions along one of the world’s most heavily militarized borders.

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Several of the North Korean soldiers were armed but many had equipment for maintenance work, South Korea’s Joint Chiefs of Staff said in a news briefing Tuesday. The JCS did not say how many shots were fired or how many soldiers breached the divide.

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. The move also appeared to be in response to South Korean activists earlier in May sending balloons into North Korea that contained anti-Pyongyang messages.

“Our assessment is that they did not have an intention for intrusion given that they immediately moved back north after our warning broadcast and warning shots,” JCS spokesperson, Lee Sung-jun, said.

More than 20 soldiers crossed the line for a short period of time, entering a few dozen yards into South Korean territory, Yonhap News said, citing a JCS official who was not identified.

Sunday’s incident took place shortly before North Korean official media quoted the sister of leader Kim Jong Un as saying Pyongyang planned reprisals against South Korea after it resumed loudspeaker broadcasts over the weekend for the first time in years.

South Korea suspended a 2018 agreement with North Korea aimed at reducing military tensions due to the balloons. The accord, once seen as a landmark in reconciliation, resulted in both sides destroying 10 front-line guard posts, enforcing bans on military exercises in the area and imposing a no-fly zone.

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.

South Korea’s Defense Ministry declined to comment on when North Korean troops last crossed the border. In 2020, North and South Korea exchanged fire after North Korean troops fired at South Korean military posts, which Seoul described as an accident — not an intended military action.

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.

–With assistance from Seyoon Kim.

(Updates with comments from South Korea’s military, details.)

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