Seoul's military said Friday it had fired warning shots after North Korean soldiers briefly crossed the heavily fortified border in the third such incursion this month.
The nuclear-armed North has been reinforcing the border in recent months, adding tactical roads and laying more landmines, which has led to "casualties" among its troops due to accidental explosions, South Korea has said.
On Thursday morning, "several North Korean soldiers who were working inside the DMZ on the central front line crossed the Military Demarcation Line," Seoul's Joint Chiefs of Staff said.
"After our military's warning broadcasts and warning shots, the North Korean soldiers retreated back northward," they added.
Similar incidents took place on June 9 and Tuesday this week, with Seoul's military saying both incursions appeared to be accidental.
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.
"While attention is focused on Putin's pariah partnerships, the Kim regime is recklessly endangering soldiers with rushed construction work at the inter-Korean border," said Leif-Eric Easley, a professor at Ewha University in Seoul.
The work is likely aimed "as much at keeping their countrymen in as it is at keeping the South Koreans out," he said, but warned that "a lack of inter-Korean communication channels and confidence-building mechanisms increases the danger of escalation in border areas."
The two Koreas have also been locked in a tit-for-tat "balloon war", with an activist in the South confirming Friday that he had floated more balloons carrying propaganda north.
The move is likely to trigger a response from Pyongyang, which has already sent more than a thousand of its own balloons carrying trash southward.
Kim's powerful sister Kim Yo Jong, a top regime spokesperson, said Friday that "dirty tissues and items" had been detected near the border after the launches and warned the North was likely to respond.
"It's obvious that something will happen now that they did something we clearly told them not to do," she said in a statement carried by the official Korean Central News Agency.
Legally, Seoul cannot prevent activists from sending balloons across the border due to a 2023 court ruling that bans represent 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.
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.
Activist Park said there was a "bit of friction" with city officials in Paju -- the area where the balloons were launched -- but he vowed not to call off his campaign unless Kim Jong Un "apologises" for sending trash.
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.
