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South Korea has confirmed that its soldiers fired warning shots after North Korean troops crossed the border on Sunday.

The incident took place in the Demilitarized Zone (DMZ), where a small group of North Korean soldiers entered South Korea at around 12:30 local time (05:30 GMT), carrying field tools such as pickaxes. According to Seoul’s military, these troops were part of a larger group of about 20 North Korean soldiers in the border area. Upon hearing the warning shots from the South Korean soldiers, the North Korean troops quickly retreated.

Seoul’s military explained that the dense vegetation and hidden border markers in the area likely contributed to the accidental crossing. “Inside [the border area] the vegetation is overgrown, and the border markers are hidden. There are no paths, and they were wading through the overgrowth,” a military spokesperson stated.

In the weeks leading up to this incident, tensions between the two Koreas have been escalating. North Korea has been launching balloons filled with trash into South Korean border towns, prompting Seoul to respond by broadcasting propaganda and K-pop music via loudspeakers. Additionally, activists in South Korea have been sending their own propaganda balloons into the North. Despite the recent border breach, there was no further movement from North Korean troops in the DMZ following their retreat on Sunday.

On Monday, Kim Yo Jong, the sister of North Korean leader Kim Jong Un, issued a stern warning to South Korea, threatening “new counteractions” if Seoul continues its loudspeaker broadcasts and does not stop activists from sending propaganda balloons.