South Korea spotted another 350 North Korean balloons “presumed” to be carrying waste on Monday, reigniting a tit-for-tat exchange as tensions on the Korean Peninsula continue to heighten.

South Korea’s Joint Chiefs of Staff said a suspected balloon was seen flying in northern Gyeonggi Province, which borders Seoul, around 9 p.m. local time (8 p.m. ET).

About 100 of the balloons fell inside South Korean regions, most of which were found in north of Gyeonggi Province and capital Seoul with “no hazardous substances” found so far, it said.

Seoul’s Metropolitan government later sent out a push notification alerting the city that a North Korean balloon had entered the sky over Seoul, adding that citizens should refrain from touching downed balloons and report any sightings to authorities.