North Korean leader Kim Jong Un's powerful sister on Wednesday mocked the "goblins of liberal democracy" in Seoul for complaining about balloons of waste apparently including faeces sent over the border, and pledged more could follow.

South Korean media shared photographs of exploded white balloons bearing garbage bags full of trash and what appeared to be excrement, after Seoul's military blasted Pyongyang for the "low class" stunt.

The North had warned over the weekend that it would shower border areas in "mounds of wastepaper and filth" to punish Seoul.

Kim Yo Jong -- who is the leader's sister and one of his regime's key spokespeople -- confirmed that "a large amount of waste paper and rubbish" was "scattered in the border and deep areas" of South Korea, beginning late Tuesday.

"We have tried something they have always been doing, but I cannot understand why they are making a fuss as if they were hit by (a) shower of bullets," she said in a statement carried late Wednesday by the official Korean Central News Agency.

Seoul's Joint Chiefs of Staff said that "unidentified objects believed to be North Korean propaganda leaflets have been identified in the Gyeonggi-Gangwon border area and the military is taking action".

"Citizens should refrain from outdoor activities, do not come into contact with any unknown objects, and report them to the nearest military base or police," it said in a statement sent to AFP, adding that the North's actions "clearly violate international laws".

"We sternly warn the North to immediately stop its inhumane and low-class actions," it added.

But Kim Yo Jong blasted what she called a double standard -- that the North's balloons were illegal, but similar items lofted by South Korean activists represented freedom of expression.

"Are 'freedom of expression' and 'international law' defined according to the direction in which balloons fly?" she said glibly.

She said the trash should be seen "as 'sincere presents' to the goblins of liberal democracy who are crying for the 'guarantee for freedom of expression'".

"We make it clear that we will respond to the ROK clans on case-to-case basis by scattering rubbish dozens of times more than those being scattered to us, in the future," she added, using the acronym for South Korea's official name.

Since the 1950-53 Korean War ended in an armistice, the two Koreas remain technically at war and are separated by a heavily fortified border.

South Korean activists sometimes release balloons carrying leaflets decrying Kim Jong Un's regime and money intended for people living north of the border.

Pyongyang has long been infuriated by the propaganda campaigns, possibly due to concerns that an influx of outside information in the tightly controlled society could pose a threat to Kim's rule.

North Korea has sent propaganda balloons across the border before, in 2016 for example, but their approach is a bit different this time, Cheong Seong-chang of the Sejong Institute told AFP.

"Bags filled with toilet paper, trash and Chinese batteries were found," he said.

"Also from witness statements that there was a 'distinctive smell' from the bag, it is likely they sent faeces, probably animal faeces, as well," he added.

"It's a stern message to South Korea that like the South, North Korea can send propaganda as well, and they should immediately stop doing it," Cheong said, adding the border would be "strongly controlled after this".

North Korea on Monday attempted to put a second spy satellite into orbit, but the launch ended in a mid-air explosion.

Seoul conducted drills with fighter jets hours ahead of the attempt in protest.

Kim Jong Un said Seoul's response was "recklessness", according to a KCNA report Wednesday.

Kim said "the present situation requires further bolstering up the war deterrence in every way and steadily developing the DPRK's armed forces into an entity of super-powerful and absolute strength," the report said.