North Korea sent hundreds more trash-carrying balloons over the border, Seoul's military said Monday, after Kim Jong Un's powerful sister warned of further responses if the South keeps 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 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 around 310 trash-carrying balloons overnight, with no more detected in the air by early Monday, the Yonhap news agency reported.

"The latest batch of waste-loaded balloons sent late Sunday contained scrap paper and plastic, with no toxic material detected so far," Yonhap said, citing the Joint Chiefs of Staff.

- 'Beyond our imagination' -

The statement from Kim's sister shows that "North Korea is raising its voice in order to shift the blame for the current situation to South Korea and also to justify their provocation," Kim Dong-yub, professor at the University of North Korean Studies in Seoul, told AFP.

It is likely that the cycle of escalation will continue and "North Korea will do something beyond our imagination," Kim said.

Pyongyang could do "something creative like throwing flour (which) will cause absolute panic in the South which they will be happy about," Kim added, referring to the possibility of the North faking a biological attack on the South.

The tit-for-tat balloon blitz started in mid-May when activists in the South -- including North Korean defectors -- sent dozens of missives carrying anti-Kim propaganda and flash drives of K-pop music northwards.

In response, Pyongyang has sent more than a thousand balloons carrying bags of garbage into the South, which Seoul has branded "low class", while also claiming it violates the armistice agreement that ended hostilities in the 1950 to 1953 Korean War.

In response to the balloons, Seoul tore up the 2018 military deal and resumed loudspeakers broadcasts along the border -- which have not been used since 2016, when they were turned on in retaliation for North Korea's fourth nuclear test, Yonhap said.

In 2018, during a period of improved inter-Korean relations, the leaders of the two Koreas agreed to "completely cease all hostile acts", including stopping the leaflets and broadcasts.

The South Korean parliament passed a law in 2020 criminalising sending leaflets to the North, but activists did not stop and the law was struck down by the Constitutional Court last year as an undue limitation on free speech.

The broadcasts, a tactic which dates back to the Korean War, infuriate Pyongyang, which previously threatened artillery strikes against the loudspeaker units unless they were switched off.