In Gyeonggi, a province near Seoul, a balloon caught fire on top of a residential building.
Firefighters extinguished the blaze, the Gyeonggi Northern Fire and Disaster Headquarters said.
South Korea’s military said some rubbish 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 spokesman for South Korea’s Joint Chiefs of Staff, told a briefing.
Lee said 480 balloons, mostly carrying paper and plastic, had landed in South Korea as of Thursday.
On Wednesday, North Korean balloons had landed near the heavily guarded presidential office in Seoul.