South Korea's military on Monday said it was "fully ready" to respond after North Korea ordered troops on the border to prepare to fire in an escalating 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".

It warned on Sunday that if another drone was detected, it would consider it "a declaration of war".

Seoul's military previously denied it was behind the flights, with local speculation centred on activist groups in the South, which 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 reinforced air observation posts in Pyongyang.

"Our military is closely monitoring the situation and standing fully ready for the North's provocations," Lee Seong-joon, a spokesman for the South's Joint Chiefs of Staff (JCS) told a press 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.

In a statement early on Monday, Kim Yo-jong said the drone flights were "an unpardonable, malicious challenge to our state".

She has issued three similar back-to-back statements, calling on South Korea's military to come up with measures to prevent a recurrence of alleged violations of North Korean airspace.

The JCS on Monday neither confirmed nor denied 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.

Seoul's military also said on Monday that 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. (AP)