South Korea's military said on Monday it was "fully ready" to respond after North Korea ordered troops on the border to prepare to fire in a dispute over drone flights to Pyongyang.
The nuclear-armed North has accused Seoul of flying drones over its capital to drop propaganda leaflets filled with "inflammatory rumours and rubbish", and warned on Sunday that it would consider it "a declaration of war" if another drone was detected.
Seoul's military initially denied it was behind the flights, with local speculation centred on activist groups in the South that have long sent propaganda and US currency northwards, typically by balloon.
But the North insists Seoul is officially to blame, announcing late on Sunday it had told eight artillery brigades already on war footing "to get fully ready to open fire" and also reinforced air observation posts in Pyongyang.
"Our military is closely monitoring the situation and standing fully ready," Lee Seong-joon, a spokesman for the South's Joint Chiefs of Staff (JCS), told a news briefing.
Pyongyang claims propaganda drones have infiltrated the capital's airspace three times in recent days, with leader Kim Jong Un's powerful sister threatening a "horrible disaster" unless they stop.
Kim Yo Jong said in a statement early on Monday the drone flights were "an unpardonable, malicious challenge to our state".
The JCS neither confirmed nor denied on Monday that Seoul's military was responsible for sending drones across the border, instead calling the North's claim "shameless".
"The North can't even confirm the origin of a drone in the Pyongyang sky but is placing blame on the South -- all the while keeping a shut mouth on its sending of a drone southward on 10 occasions," spokesman Lee said.
The United Nations Command, which oversees the armistice that ended active fighting in the 1950 to 1953 Korean War, said it was aware of the North Korean claim.
"The command is currently investigating the matter in strict accordance with the Armistice Agreement," it said. The two Koreas remain technically at war.
Seoul's military said on Monday the North appeared to be preparing to carry out explosions at roads connected to the South, days after Pyongyang said it would seal the border.
The North's Korean People's Army (KPA) announced last week that the measure will "completely separate" North Korea's territory from the South.
South Korean JCS spokesman Lee said it was possible the road blasts would take place "as early as today".
North Korea has been bombarding the South with trash-carrying balloons it says are in retaliation for propaganda launched by activists in the South.
Seoul's unification ministry said the drone claims may be an effort by the North to bolster internal solidarity.
Koo Byoung-sam, a spokesperson at the ministry, told a news briefing that the North could also be looking for an excuse "to stage provocations or create anxiety and confusion in our society".
One expert said it was "more likely" that the drones had been launched by activists in the South rather than fabricated by the North because Pyongyang's statements were effectively an admission that air security had been breached.
"Even if they were trying to stage this, it would expose a significant vulnerability in their skies," said Yang Uk, a research fellow at the Asan Institute for Policy Studies.
The Kim dynasty relies on its total control of information to stay in power, with most North Koreans lacking access to the internet, cell phones and outside information.
"If sending information via drones becomes a regular activity, it would be a serious issue for North Korea," Yang said.
Former National Intelligence Service chief Park Jie-won said on a radio show on Monday that the government's refusal to confirm or deny their involvement in the drones was an admission of guilt.
"The appropriate response is to say that we cannot confirm anything. In my view, this is essentially an acknowledgement," Park said.
