North Korea is sending more balloons likely to be carrying trash into the South, Seoul's military said late Monday, continuing a tit-for-tat balloon war between the two Koreas.

Seoul's Joint Chiefs of Staff said that the North "is launching (suspected) trash balloons aimed at the South once again," adding that the balloons were currently floating across the border.

"Citizens are advised to be cautious of falling debris. If you find any fallen balloons do not touch them and report them to the nearest military unit or police station," it added.

Pyongyang has already sent more than a thousand balloons carrying trash southward, as what it calls retaliation for balloons carrying anti-regime propaganda floated into the North by activists south of the border. 

An activist in the South confirmed Friday that he had launched more balloons, with Pyongyang vowing to respond.

Seoul city authorities issued an alert to residents Monday evening saying: "a trash balloon from North Korea has been confirmed to have entered Seoul's airspace."

Legally, South Korea cannot sanction activists sending balloons across the border due to a 2023 court ruling that bans it as an unjustifiable infringement on free speech.

Activist Park Sang-hak, who defected from North Korea and has been sending anti-regime leaflets north for years, said he floated 20 balloons laden with propaganda as well as flash drives with K-pop and television dramas across the border on Thursday last week.

The North is extremely sensitive about its people accessing South Korean pop culture, with a United Nations report saying possession of large amounts of such content has been known to result in the death penalty.

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

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.

Relations between the two Koreas are at one of their lowest points in years, with Kim Jong Un hosting Russian leader Vladimir Putin this week, and signing a mutual defence agreement that has raised hackles in Seoul.

In response, the South -- a major weapons exporter -- has said it will "reconsider" a longstanding policy that has prevented it from supplying arms directly to Ukraine.

Experts said it was possible that border tensions could escalate quickly.

"We also can't rule out the possibility that North Korea might take more drastic provocative actions because of their confidence after signing the treaty with Russia," said Hong Min, a senior research fellow at the Korea Institute for National Unification.

A US aircraft carrier arrived in South Korea on Saturday for joint military drills aimed to better counter North Korean threats, ahead of joint Seoul, Washington and Tokyo military drills later this month.

Pyongyang has always decried similar combined drills as rehearsals for an invasion.