North Korea has sent balloons full of trash, toilet paper and suspected animal faeces into the South, local media reports said Wednesday, with Seoul's military slamming Pyongyang for their "low class" actions.

Photographs showing white balloons bearing garbage bags full of trash and what appeared to be excrement were shared widely by South Korean media, after the North warned this weekend it would shower border areas in "mounds of wastepaper and filth" to punish Seoul.

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.

It said that the North's actions "clearly violate international laws and seriously threaten the safety of our people," saying some of the balloons had contained suspected trash, which the military was checking.

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

Late Tuesday night, Gyeonggi province issued a text message alert to residents saying: "Refrain from outdoor activities and report (objects from North Korea) to military bases when identified."

South Korean activists sometimes release balloons carrying anti-Kim Jong Un regime propaganda leaflets 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 the Kim regime.

North Korea recently threatened to retaliate.

"Tit-for-tat action will be also taken against frequent scattering of leaflets and other rubbish by the ROK near border areas," Kim Kang Il, a vice-minister of defence, said in a statement Sunday, using the acronym for South Korea's official name.

"Mounds of wastepaper and filth will soon be scattered over the border areas and the interior of the ROK and it will directly experience how much effort is required to remove them," Kim said in the statement carried by the official Korean Central News Agency.

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.

- 'Toilet paper, trash' -

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, after Pyongyang informed Tokyo of the pending launch window earlier in the week.

North Korea's Kim 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.