The latest launch of ‘trash balloons’ by Pyongyang led to flight suspensions at Seoul’s main international airport

South Korea’s military has vowed a “stern” response if North Korea “crosses the line” with its campaign of sending trash-filled balloons onto its neighbor’s territory, local media reported on Monday. The two countries have been deploying inflatables as part of their long-running confrontation.

The warning on Monday came after a barrage launched by Pyongyang between Sunday evening and early Monday morning led to flight suspensions at South Korea’s main international airport, according to NK News.

The spokesperson for Seoul’s Joint Chiefs of Staff (JCS), Col. Lee Sung-jun, said at a press briefing that the military viewed such incidents as “having a higher level of risk,” the Yonhap news agency reported.

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“If serious danger is caused to the safety of our citizens or if the North is assessed to have crossed the line, our military will take stern military measures,” Lee was quoted as saying.

The spokesperson declined to elaborate when asked what such a “line” would be. The military does not consider the balloons a serious threat to public safety “as long as there are no fatalities,” he added, according to NK News.

Some of the ballons sent by the North are equipped with timer devices that “could potentially cause fires,” Yonhap noted.

North Korea began sending trash-filled balloons toward its neighbor in May. Pyongyang says it has resorted to such unconventional tactics in retaliation for balloons being sent to the North carrying propaganda leaflets. Seoul, in turn, responded by resuming full-scale daily propaganda broadcasts through its border loudspeakers in July.

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Tensions between the two neighbors have been running high in recent months in light of Pyongyang’s missile tests and Seoul’s joint drills with the US on the peninsula.