On Friday, North Korea accused South Korea of deploying unmanned drones to drop propaganda leaflets over Pyongyang, its capital, and warned of potential military retaliation if such flights persist.

According to a Friday statement from North Korea’s Foreign Ministry, South Korean drones were spotted over Pyongyang on Wednesday and Thursday night. These drones reportedly released “numerous leaflets filled with political propaganda and insults” aimed at North Korean leader Kim Jong-un’s regime.

North Korea labeled the incidents as “serious political and military provocations” that could escalate into “armed conflict.” It stated that its military is preparing “all possible measures of attack” and would respond immediately and forcefully if South Korean drones are detected in its airspace again.

“The culprits should stop putting their citizens’ lives at risk,” the statement warned.

No anti-North Korean activist group in South Korea has taken responsibility for the drone operations. The South Korean military has neither confirmed nor denied the North’s allegations, but cautioned North Korea against any hasty actions. Notably, North Korea’s statement did not specify the type of drones that were allegedly observed.

In response, South Korea declared, “We will respond decisively and without hesitation if the North threatens the safety of our people.”

Tensions between the two Koreas have been rising recently, partly due to anti-North Korean activists in South Korea, mainly defectors, who have been sending balloons filled with leaflets criticizing Kim Jong-un’s regime across the border. Since May, North Korea has also released thousands of balloons toward the South, though these have primarily carried scrap paper and household waste.

North Korea’s aggressive rhetoric toward the South has intensified following the collapse of Kim’s diplomatic efforts with former U.S. President Donald J. Trump in 2019. Their talks, aimed at reducing North Korea’s nuclear weapons program in exchange for easing U.N. sanctions, ultimately fell apart.

Since then, Kim has increased weapons testing, while South Korea has strengthened its military alliances with the U.S. and Japan.

During the Cold War, both North and South Korea frequently used balloons to send propaganda across the border. However, during the first inter-Korean summit in 2000, both countries agreed to end these state-sponsored campaigns. Despite this, North Korean defectors and conservative Christian groups in South Korea have continued the practice, using balloons to send mini-Bibles, USB drives containing K-pop, and leaflets criticizing Kim Jong-un, even calling him a “pig.”

The North has condemned the leaflets, referring to them as political “filth.”