Kim Jong Un's sister has threatened retaliation and said claims North Korea planted land mines that injured three South Korean soldiers are a "conspiratorial farce".
They were hurt in two explosions on 21 September in the western section of the heavily fortified buffer zone between the countries, known as the DMZ.
Kim Yo Jong hit out after South Korea's military announced the preliminary results of an inspection conducted with the US-led United Nations Command.
It said a follow-up on Tuesday had found another North Korean mine.
The UN Command accused North Korea of breaching the armistice deal that prohibits any hostile act "within, from, or against" the DMZ.
The incident could increase tensions between the neighbours, who do not have working military hotlines or other emergency communication channels as North Korea hardens its stance towards Seoul.
South Korea's military officials called for an apology and said they wanted the North to take "responsible measures" to prevent a recurrence.
Defence minister Kang Shin-chul also warned of "corresponding measures" if the full investigation confirmed the North was to blame.
But Kim Yo Jong - considered the second most powerful person in North Korea - accused the South of making groundless claims as a pretext for provocation against her country.
"It is too low and dirty as befitting the tribes of South Korea," she said in a statement on state media.
She denied claims the North's soldiers briefly crossed the border during fortification work and claimed South Korea had fired warning shots.
"If the (South Korean) military gangsters fire at our soldiers carrying on the permanent project for the southern border, not warning shots, our border units will take immediate and merciless retaliatory action," she warned.
"If a single bullet hits our land, they must be prepared for a lot," she added.
The North has added mines, anti-tank barriers and other fortifications to its side of the border in recent years, raising concerns in the South about increased troop numbers in the zone.
About two million mines are already estimated to litter the DMZ, which is about 2.5 miles wide and 155 miles long.
The zone itself is officially demilitarised and has become something of a haven for wildlife, but the area either side is one of the most heavily fortified border areas in the world.
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The South Korean soldiers were injured about 10m south of the military demarcation zone - essentially the land border that runs through the middle of the DMZ - during an operation to establish a new patrol route.
The route is intended to allow more inspections after "terrain changes", potentially from mine laying, were detected last year.
In 2015, tensions also spiked when two South Korean soldiers were maimed following a series of mine blasts, but talks eventually pulled the two sides back from the brink.
(c) Sky News 2026: Kim Jong Un's sister issues warning as she denies South Korea land mine claims
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