South Korean troops fired warning shots after North Korean soldiers briefly crossed the border this week, Seoul's military said on Tuesday, with tensions high over Pyongyang's barrage of trash-carrying balloons and the South's retaliatory loudspeaker campaign.

The Sunday incursion over the line that separates the two militaries took place in an overgrown area of the heavily fortified border area and was likely accidental, the Joint Chiefs of Staff said.

Relations between the two Koreas -- technically still at war as the 1950-1953 conflict ended in an armistice rather than a peace treaty -- are at one of their lowest points in years.

"Some North Korean soldiers working within the DMZ on the central front briefly crossed the Military Demarcation Line (MDL)," the JCS said in a statement, referring to the line of control between the two Koreas.

"After our military issued warning broadcasts and warning shots, they retreated northward," it said, adding there had been "no unusual movements observed" subsequently.

About 20 North Korean soldiers crossed the border, according to the Joint Chiefs of Staff. 

The incursion was likely accidental, JCS spokesperson Lee Sung-joon told reporters Tuesday.

"The situation at that time was that the DMZ was now overgrown with trees and the MDL mark was not clearly visible," Lee said. 

"There was no road, and the (North Korean soldiers) were moving through the bushes, and we were observing (them) even before they got close to the MDL.

"We believe that they did not intend to invade, considering that they immediately moved northward after the warning broadcasts and warning shots."

In recent weeks, North Korea has sent more than a thousand balloons laden with trash including cigarette butts and toilet paper southward -- a response, it says, to balloons carrying anti-Pyongyang propaganda sent north by activists.

In response, the South Korean government has suspended a 2018 tension-reducing military deal and restarted loudspeaker propaganda broadcasts along the border, infuriating the North, which warned Seoul was creating "a new crisis".

North Korea could be re-installing its own loudspeakers along the border, Seoul's military said Monday, a tactic it had used since the 1960s but suspended in 2018.

Seoul's spy agency told AFP on Tuesday it had also detected signs that Pyongyang was demolishing sections of the inter-Korean railway.

'Small provocation'

The incursion of the North Korean soldiers could be a "small provocation" to test the waters ahead of a bigger move, Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.

"It can also be seen as part of (chief regime spokeswoman and sister of Kim Jong Un) Kim Yo Jong's preparation for what she described as a 'new countermeasures'" he added.

Pyongyang has previously threatened artillery strikes against the loudspeaker units.

South Korea's loudspeaker broadcast on Sunday included news segments about Seoul's decision to suspend the 2018 military agreement along with a report on the global sales performance of Samsung Electronics smartphones, according to the Yonhap news agency. 

It also played songs by K-pop sensation BTS, Yonhap said. 

Apart from anti-Kim Jong Un leaflets, North Korea is also extremely sensitive about its people gaining access to South Korea's flourishing popular culture. 

According to a United Nations report, Pyongyang enacted a law in 2020 to punish anyone possessing or distributing a large amount of South Korean media content with life imprisonment or even the death penalty.

Experts have warned that the decision to jettison the 2018 deal and restart loudspeaker broadcasts could have serious implications, as previous tit-for-tat propaganda actions have had real-world consequences for inter-Korean relations.

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