Relations between the two Koreas are at a low point, with the North's leader Kim Jong Un declaring South Korea to be his country's "principal enemy" earlier this year.

Citing the North's defence ministry statement, the official Korean Central News Agency reported that "the General Staff of the KPA issued a preliminary operation order on October 12 to the combined artillery units along the border... to get fully ready to open fire."

KPA stands for the Korean People's Army, the North's military.

The order was for "eight artillery brigades fully armed at full wartime strength on standby to open fire" until Sunday night at 8 pm (1100 GMT).

KCNA also said other units were ordered to "intensify monitoring on full alert" while "anti-air observation posts have been reinforced" in the capital city Pyongyang.

North Korea on Friday said the South had sent drones carrying propaganda leaflets into Pyongyang's airspace on October 3, and then again on Wednesday and Thursday last week.

The drones allegedly sent by the South had dropped anti-regime propaganda, and the leaflets were filled with "inflammatory rumors and rubbish", KCNA said.

Flying drones into Pyongyang's airspace "could be considered a military attack", the North's foreign ministry said, according to KCNA, adding it is "an intolerable and unforgivable grave provocation".

The South Korean defense minister, Kim Yong-hyun, initially denied the claim, but the country's Joint Chiefs of Staff later amended that position, saying in a statement that they "cannot confirm whether the North Korean allegations are true or not".

The North's army last week said it would "permanently shut off and block the southern border" with South Korea.