North Korea has resumed sending balloons carrying trash over the border to South Korea, officials and news reports said on Sunday, a week after it vowed to continue if anti-North Korea leaflets are flown from the South.

Dozens of balloons with trash attached have been found in Seoul and in areas near the border overnight and early on Sunday, after the South Korean military said late on Saturday the North was again launching them.

South Korea’s military said on Sunday it takes the balloons “very seriously” and was on alert to take action in response as necessary. It did not elaborate what actions it would take.

South Korea has warned it would take “unendurable” measures against the North for sending the trash balloons, which could include blaring propaganda broadcasts from huge loudspeakers set up at the border directed at the North.

The North Korean government has said the balloons were sent in retaliation to anti-North leaflets flown by South Korean activists as part of a propaganda campaign and launched hundreds of them starting late in May carrying trash and manure.

On June 2, it said it would temporarily halt sending the balloons because 15 tons of trash it sent was probably enough to get the message across how “unpleasant” it was. However, it vowed to resume if leaflets are again flown from the South by sending hundred times the amount.

A group of South Korean activists defied the warning and have since flown more balloons to the North with leaflets criticising its leader Kim Jong Un together with USB sticks containing K-pop videos and dramas, and U.S. dollar notes.

North Korea has shown some of the angriest reaction towards the leaflet campaign and the loudspeaker broadcasts, in some cases firing weapons at the balloons and the speakers in the past.

Experts have said the reaction was an indication Pyongyang considers the propaganda as a serious threat to controlling its public.