North Korea has sent about 500 balloons laden with trash into South Korea’s air space over the past 24 hours, officials in the South said on Thursday, disrupting flights and igniting a fire on the roof of a residential building.

The balloons are part of an ongoing propaganda campaign by Pyongyang against North Korean defectors and activists in the South, who regularly send balloons carrying items such as anti-Pyongyang leaflets medicine, money and USB sticks loaded with K-pop videos and dramas.

A suspected balloon suspended take-offs and landings at Seoul’s Gimpo Airport on Wednesday evening for two hours, an official at the Korea Airports Corporation said. Balloons have affected traffic at South Korea’s main international airport, Incheon, several times in recent weeks.

In Gyeonggi, a province near Seoul, a balloon caught fire on top of a residential building. Fire fighters extinguished the blaze, an official at the Gyeonggi Northern Fire and Disaster Headquarters said.

South Korea’s military said some trash balloons were equipped with timed poppers that could cause fires.

“A timer is attached to the trash balloons, which has the effect of popping the balloons and spreading the trash after a certain period of time has passed,” Lee Sung-jun, a spokesperson for South Korea’s Joint Chiefs of Staff, told a briefing.

Lee said 480 balloons had landed mostly carrying paper and plastic trash in South Korea as of Thursday.

On Wednesday, North Korean balloons had landed in the vicinity of the heavily guarded presidential office in Seoul.