South Korea has warned residents living near the border with North Korea to be on alert after media reports said more than 90 balloons carrying various objects including what appeared to be trash and excrement had been detected.
South Korea's military said unidentified objects believed to be from North Korea were seen near the heavily fortified border and urged people to keep away and report them to the military or police.
By Wednesday, more than 90 balloons had been detected, with some landing on the ground, while others were still in the air, Yonhap News reported, citing a military source.
Photographs published in South Korean media showed an inflated white balloon with a plastic bag tethered to it and in another image what appeared to be a collapsed balloon with trash strewn around it.
North Korea's vice defence minister issued a statement on Sunday vowing to exercise "strong power for self-defence" and warned that "mounds of waste-paper and filth" would be sent to the South in response to its flying "dirty things" to the North.
For years, South Korean activists have sent balloons to North Korea carrying leaflets criticising its leaders and USB memory sticks of K-pop music videos.
North Korea and South Korea remain technically at war since an armistice agreement ended fighting in the 1950-1953 Korean War.
Their large militaries face off across the inter-Korean border and North Korea has deployed missiles and rockets aimed at the South and routinely threaten to annihilate its neighbour.
Peter Ward, research fellow at the Sejong Institute, said sending balloons was a far less risky than taking overt military action.
"These kinds of grey zone tactics are more difficult to counter and hold less risk of uncontrollable military escalation, even if they're horrid for the civilians who are ultimately targeted," he said.
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