Kim Yo Jong, the influential sister of North Korean leader Kim Jong Un, said the country would raise its alert status along the entire southern border, according to the state-run Korean Central News Agency (KCNA).
She said anti-South Korean loudspeakers would be ready for deployment along the border within days, signaling a further escalation in tensions between the two neighbors.
Kim warned that continued interference by Seoul would trigger severe consequences, although the specific actions that prompted her latest warning were not immediately clear.
The announcement marks another deterioration in relations between the two Koreas, which remain technically at war because the 1950-53 Korean War ended in an armistice rather than a peace treaty.
The latest warning comes amid tensions over a Sept. 21 land mine explosion in the Demilitarized Zone that wounded three South Korean soldiers, two seriously.
Seoul accused Pyongyang of deliberately planting the mines and sought an apology, while North Korea rejected the allegations as "blame-shifting" and accused the South of attempting to tarnish its international image.