North Korea on Saturday sent a new round of trash-laden balloons towards the South, Seoul's military said, after anti-Pyongyang activists in the South said they had lofted balloons with leaflets against leader Kim Jong Un across the border.

The announcement came hours after Seoul's military said it was on alert for a new filthy salvo arriving from North Korea, as the tit-for-tat balloon blitz revved up once again.

"North Korea is again floating (suspected) balloons carrying trash towards the South," the Joint Chiefs of Staff said in a statement, advising the public to report any balloons to authorities and refrain from touching them.

The Seoul city government, as well as officials in Gyeonggi province, also sent a similar text alert to residents on Saturday, warning about the balloons.

North Korea sent hundreds of balloons in two waves last week with bags of trash into the South, describing them as retaliation for anti-Pyongyang propaganda balloons sent the other way.

Pyongyang said it would stop last Sunday but days later, a South Korean group called "Fighters for Free North Korea" said it had sent 10 balloons with thumb drives containing K-pop music and 200,000 leaflets condemning Kim's rule.

Another group, comprising North Korean defectors, also said it had sent 10 balloons on Friday with 100 radios, 200,000 anti-Pyongyang leaflets, and thumb drives containing a speech by South Korean President Yoon Suk Yeol.

Jang Se-yul, the leader of the second group, told AFP on Saturday that his organisation would not stop its balloon campaign, "whether Kim Jong Un sends trash-carrying balloons again or not".

Last year, South Korea's Constitutional Court struck down a 2020 law that criminalised the sending of anti-Pyongyang propaganda, calling it an undue limitation on free speech.

Experts say there are now no legal grounds for the government to stop activists from sending balloons into North Korea.

South Korea's unification ministry said Saturday that the issue is "being approached in consideration" of last year's court ruling.

Kim's powerful sister Kim Yo Jong mocked South Korea for complaining about the balloons last week, saying North Koreans were simply exercising their freedom of expression.