The sister of North Korean leader Kim Jong-un on Monday warned of further responses if the South kept up its "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 influential sister and key government spokeswoman Kim Yo-jong said in a statement released 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. (AFP)