North Korea sent hundreds more trash-carrying balloons to South Korea, Seoul's military said Monday, following a warning from Kim Jong Un's sister about further responses to the South's "psychological warfare".

In recent weeks, North Korea has sent hundreds of balloons into the South, carrying trash like cigarette butts and toilet paper, in what it calls retaliation for balloons laden with anti-Pyongyang propaganda floated northwards by activists in the South, which Seoul legally cannot stop.

The South Korean government this month fully suspended a 2018 tension-reducing military deal and restarted loudspeaker propaganda broadcasts along the border in response to Pyongyang's balloons, infuriating the North which warned Seoul was creating "a new crisis."

Kim's sister and key government spokeswoman Kim Yo Jong said in a statement released early Monday that South Korea would "suffer a bitter embarrassment of picking up waste paper without rest and it will be its daily work."

In the statement carried by the official Korean Central News Agency, she slammed the activists' leaflets as "psychological warfare" and warned that unless Seoul stopped them and called off the loudspeaker broadcasts, the North would hit back.

"If the ROK simultaneously carries out the leaflet scattering and loudspeaker broadcasting provocation over the border, it will undoubtedly witness the new counteraction of the DPRK," she said, referring to both countries by their official names.

Seoul's military said the North had sent more than 300 trash-carrying balloons overnight, but that the winds had not worked in Pyongyang's favor.

"Although they launched over 310 balloons many of them flew toward North Korea," the Joint Chiefs of Staff said, adding that around 50 had landed in the South so far, with more expected.

They said that the latest batch of trash balloons had been found to contain waste paper and plastic, but nothing toxic.

"So far we haven't seen any special movement within the North Korean military," a JCS official said, adding that they had "detected signs of North Korea installing loudspeakers to broadcast to the South in the front area."

North Korea has also used its own loudspeakers along the border since the 1960s, typically broadcasting praise of the Kim family, but it suspended them in 2018 as ties warmed