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…