SEOUL, Sept 16, 2024 (BSS/AFP) - A trash balloon from North Korea landed on a Seoul building rooftop and caused a fire, a local fire department said Monday, among thousands of balloons Pyongyang has sent southward this year, sparking a tit-for-tat propaganda campaign.

"At around 9:04 pm (1204 GMT) on Sunday, a fire broke out on the rooftop of a four-story commercial building in the Western district of Seoul," Seoul's Gangseo Fire Station said in a statement.

The fire was extinguished in 18 minutes and 15 fire trucks and 56 personnel were deployed, said the fire department, adding there were no casualties.

Military and police authorities collected the balloon, which was being investigated.

Pyongyang launched about 120 trash-filled balloons towards the border late Sunday, following 50 balloons that were sent on Saturday, South Korea's military said Monday.

Around 40 of the balloons had already landed in the South, mainly in northern Gyeonggi province and the capital Seoul, the Joint Chiefs of Staff (JCS) said.

The bags attached to those balloons contained "mostly paper and plastic waste", the JCS said, adding that they posed no safety risk to the public.

North Korea has sent more than 5,000 trash-filled balloons southward since May, saying they are retaliation for propaganda balloons launched by South Korean activists.

In response, Seoul has suspended a tension-reducing military deal with Pyongyang and restarted some propaganda broadcasts from loudspeakers along the border.

Timer devices on trash balloons caused fires earlier this month, with one near an airport and another at a storage unit.

"Some North Korean trash balloons have thermal timers that could potentially cause fires if they don't separate properly when the heating wire is activated, which acts to detach the balloon from its load," JCS spokesperson Lee Chang-hyun told reporters.

"We have seen it being described as a 'timer explosion', but we would like to explain again that the method is where a thermal timer heats the balloon's material, causing it to tear apart in mid-air," he added.

Lee also said "shooting down the balloons in the air increases the risk of falling debris or hazardous materials, so for now, the safest approach is considered to be quickly collecting them after they fall naturally".

North Korea likely has data to "predict where the balloon will land after flying for a certain number of hours and then detonate the device mid-air to where they intend to drop it," Yang Uk, a research fellow at the Asan Institute for Policy Studies, told AFP.

Although no one has been hurt and there has been no damage, "the balloon fires can lead to worse when the season becomes dry", Yang added.