This photo provided by the Gyeonggi Provincial Fire Service, shows the ceiling of a storehouse damaged by a fire after a balloon sent by North Korea hit it in Paju, Gyeonggi Province, Sept. 8. Yonhap

Timer devices on some North Korean trash-carrying balloons could cause fires, South Korea's military said Tuesday, amid a series of fires in the country suspected to be caused by the balloons.

On Monday, firefighters put out a blaze that began on the rooftop of a warehouse in Paju, 27 kilometers northwest of Seoul, after a North Korean trash balloon landed there, renewing concerns whether the balloons or the substances they drop could be a fire hazard.

A Joint Chiefs of Staff spokesperson said some North Korean balloons have dropped bundles of trash using devices that heat up and disconnect the garbage after a certain period of time, noting they could lead to fires if they land on surfaces without properly separating.

"We believe there is a possibility of a fire when the thermal wires are activated to separate the balloons from their load," Lt. Col. Lee Chang-hyun told a regular briefing. "The exact cause of such fires is being investigated by relevant authorities."

Lee still reaffirmed the military's policy against shooting down the balloons, citing potential risks of more damage caused by debris from intercepted balloons.

The defense ministry's spokesperson Jeon Ha-kyou said the military is working with the police to come up with preventative measures to minimize damage from the balloons.

Since late May, North Korea has launched thousands of trash-carrying balloons toward the South in retaliation against anti-Pyongyang leaflets sent across the border by North Korean defectors and activists in South Korea. (Yonhap)