North Korean leader Kim Jong Un's powerful sister on Sunday denied Seoul's claims that Pyongyang had fired dozens of artillery rounds near their border a day earlier.

On Saturday, Seoul's military said North Korean forces had fired over 60 artillery rounds near Yeonpyeong Island, a day after both sides staged live-fire drills in the same area near their contested maritime border.

The South's Joint Chiefs of Staff said the shells landed in a buffer zone created under a 2018 tension-reducing deal. That deal fell apart in November after the North launched a spy satellite.

"Our military did not fire a single shell into the water area," Kim Yo Jong said in a statement carried by the official Korean Central News Agency.

Kim claimed instead that her country's military had detonated explosives simulating the sound of gunfire 60 times and "watched the reaction" of the South Korean forces.

"The result was exactly as we expected," she said, adding: "They misjudged the sound of explosives as gunfire, assumed it was an artillery fire provocation, and shamelessly made up a lie."

"In the future, they will misjudge even the rumbling sound of thunder in the northern sky as artillery fire from our military," she said.

Residents on two South Korean islands near the border were ordered to evacuate on Friday after the North fired more than 200 artillery shells near Yeonpyeong and Baengnyeong during the live-fire exercises.

Relations between the two Koreas are at one of their lowest points in decades after leader Kim Jong Un last year enshrined his country's status as a nuclear power into the constitution and test-fired several advanced ICBMs.

At Pyongyang's year-end policy meetings, Kim threatened a nuclear attack on the South and called for a build-up of his country's military arsenal ahead of armed conflict that he warned could "break out any time".