Trash from a balloon sent by North Korea was found in a government complex in Seoul on Friday morning, more than a day after one of the North’s balloon launches was last detected, adding to concerns about South Korea’s potential vulnerability to the provocative new tactic.

       Military officials said they had recovered the debris from the parking lot of the complex, which houses various ministries in the heart of Seoul, a city of 10 million. They said it had probably come from a balloon launch that the military detected by radar on Wednesday evening, and that it was unclear when it had landed on the compound.

       It was at least the third time that debris from one of the North’s balloons had been found in a South Korean government complex.

       “The fact that the trash was found there a few days after the launch is concerning from a security perspective,” said Kim Yong-hyun, a professor of North Korean studies at Dongguk University in Seoul. “It would be a whole different game if they weren’t trash but biological weapons or hazardous materials.”

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       Since May, North Korea has launched thousands of balloons tied to bags of trash, about 10 percent of which have reached South Korean soil, Mr. Kim said. The North has called it a response to South Korean activists who have been sending propaganda into the North by balloon for years.

       South Korea, which technically has been at war with the North for decades, has advanced armaments and exports weapons to other countries. But it has been frustrated and stymied by North Korea’s low-tech balloon tactic, though so far it has essentially been a nuisance, with no hazardous material found in any of the payloads.

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