The powerful sister of North Korean leader Kim Jong Un vowed on Sunday to respond to what she called a fresh South Korean civilian leafleting campaign, signalling that Pyongyan will soon resume flying rubbish-carrying balloons across the border.

Since late May, North Korea has floated numerous balloons carrying waste paper, scraps of cloth, cigarette butts and even manure towards South Korea on a series of late-night launch events, saying they were a tit-for-tat action against South Korean activists scattering political leaflets via their own balloons.

No hazardous materials have been found.

South Korea responded by suspending a 2018 tension-reduction deal with North Korea and resumed live-fire drills at border areas.

In a statement carried by state media, Kim Yo Jong said “dirty leaflets and things of (the South Korean) scum” had been found in border and other areas of North Korea on Sunday morning.

“Despite the repeated warnings of (North Korea), the (South Korean) scum are not stopping this crude and dirty play,” she said.

“We have fully introduced our countermeasure in such situation. The (South Korean) clans will be tired from suffering a bitter embarrassment and must be ready for paying a very high price for their dirty play.”

North Korea last sent rubbish-carrying balloons towards South Korea in late July.

It was not immediately known if, and from which activists’ group in South Korea, balloons were sent to North Korea recently. For years, groups led by North Korean defectors have floated huge balloons carrying anti-Pyongyang leaflets, USB sticks containing K-pop songs and South Korean drama, and US dollar bills towards North Korea.

Experts say North Korea views such balloon campaigns as a grave provocation that threaten its leadership because it bans official access to foreign news for most of its 26 million people.

On June 9, South Korea redeployed gigantic loudspeakers along the border for the first time in six years, and resumed anti-North Korean propaganda broadcasts.

South Korean officials say they do not restrict activists from flying leaflets to North Korea, in line with a 2023 constitutional court ruling that struck down a contentious law criminalising such leafleting, calling it a violation of free speech.

Kim Yo Jong’s statement came a day after North Korea’s Defence Ministry threatened to bolster its nuclear capability and make the US and South Korea pay “an unimaginably harsh price” as it condemned its rivals’ new defence guidelines that it says reveal an intention to invade the North.