North Korea sent hundreds of balloons in two waves last week with bags of trash into the South, describing them as a response to anti-Pyongyang propaganda balloons sent the other way by South Korean activists.

Pyongyang announced a halt to the balloons on Sunday but days later, a South Korean group called "Fighters for Free North Korea" said it had sent 10 balloons with USB thumb drives containing K-pop music and 200,000 leaflets against leader Kim Jong Un.

The South Korean military is "closely monitoring with vigilance" because of "the possibility of more trash balloons descending around tomorrow", its spokesperson told AFP on Saturday.

North Korea had said it would respond with "wastepaper and rubbish" a hundred times the amount if more South Korean leaflets were sent.

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

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

The North Korean balloons last week landed in a number of locations in the South, and were found to be carrying garbage such as cigarette butts, cardboard scrap and waste batteries.

In response to the balloons, South Korea on Tuesday completely suspended a 2018 military deal with the North, which was meant to reduce tensions between the neighbours.

Authorities in Seoul have condemned the North Korean balloons as a "low-class" act and threatened countermeasures that it said Pyongyang would find "unendurable".

- Duelling propaganda -

Activists in South Korea have long sent balloons northwards, filled with anti-Pyongyang propaganda, cash, rice, and Korean TV series on USB thumb drives.

These have always infuriated North Korea, whose government is extremely sensitive about its people gaining access to South Korean pop culture.

Kuensaem, another South Korean activist group, told AFP that it threw 500 plastic bottles into the sea on Friday near the border with North Korea.

The bottles were filled with rice, cash and a USB drive with a South Korean TV series "Crash Landing on You" -- which features a romance between a wealthy South Korean heiress and a North Korean army officer.

"We were just doing what we've been doing for a long time to help North Koreans who are starving," the group's leader Park Jung-oh told AFP Saturday.

Tensions over the duelling propaganda have boiled over in dramatic fashion in the past.

In 2020, blaming the anti-North leaflets, Pyongyang unilaterally cut off all official military and political communication links with Seoul and blew up a disused inter-Korean liaison office on its side of the border.

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 on Saturday that the issue is "being approached in consideration" of the 2023 court decision.

Kim Jong Un'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.