The powerful sister of North Korean leader Kim Jong Un vowed Sunday to respond to what she called a fresh South Korean civilian leafleting campaign, signaling North Korea would soon resume flying trash-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 toward 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 that “dirty leaflets and things of (the South Korean) scum” were found again in border and other areas in 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,” Kim Yo Jong said.

North Korea last sent rubbish-carrying balloons toward South Korea in late July. It wasn’t 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 U.S. dollar bills toward North Korea.

Experts say North Korea views such balloons campaigns as a grave provocation that can 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 don’t restrict activists from flying leaflets to North Korea, in line with a 2023 constitutional court ruling that struck down a contentious law criminalizing such leafleting, calling it a violation of free speech.

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