North Korea resumed flying balloons likely carrying trash toward South Korea on Thursday, the South's military said, days after it vowed to respond to what it called repeated South Korean civilian leafleting campaigns in the North.

South Korea's Joint Chiefs of Staff said in a statement that the balloons were flying north of Seoul, which is about an hour's drive from the border, on Thursday afternoon.

It warned the South Korea public to be cautious of falling objects and report to authorities if they spot balloons on the ground.

Beginning in late May, North Korea has floated numerous balloons carrying wastepaper, scraps of cloth, cigarette butts and even manure toward South Korea, saying they were in response to South Korean activists sending political leaflets to the North via their own balloons. No hazardous materials were found.

In response, South Korea suspended a 2018 tension-reduction deal with North Korea, resuming propaganda broadcasts briefly and front-line live-fire military drills at border areas.

Earlier this week, the powerful sister of North Korean leader Kim Jong Un vowed to respond to what she called new South Korean civilian leafleting activities. She said South Korean balloons have been found frequently at the North Korean border and other areas.

In her statement Tuesday, Kim Yo Jong threatened new retaliatory steps, saying South Korean “scum” must be ready to pay “a gruesome and dear price.” That raised concerns that North Korea could stage physical provocations, rather than balloon launches.

South Korea's military said Wednesday it has boosted its readiness to brace for any provocation by North Korea. It said North Korea may fire at incoming South Korean balloons across the border.