Kim Yo-Jong, close ally of the Korean dictator, said attacks would be launched on the South in response to even the “slightest provocation”.
Her disturbing comments on Sunday follow several days of shelling into the sea north of the border dubbed the biggest escalation in years.
Seoul accused Pyongyang of blasting more than 200 shells towards Yeonpyeong and Baengnyeong islands, close to the countries’ sea border.
The south condemned the bombardment as a “provocative act that threatens peace”.
Local residents were told to evacuate and ferries were suspended.
After the initial firing of hundreds of artillery shells on Friday morning, South Korean authorities issued evacuation orders.
They said the North fired a further 60 rounds on Saturday near the disputed border, followed by more than 90 on Sunday.
Kim said: “I make myself clear once again that the safety catch of trigger of the Korean People’s Army (KPA) had already been slipped.
“As already declared, the KPA will launch an immediate military strike if the enemy makes even a slight provocation.”
Younger than her supreme leader brother by four years, Kim is considered one of the most powerful members of the North Korean regime.
South Korea’s Yonhap news agency said they had no plans to fire back following Saturday’s shelling but they did hold fire drills in the sea on Friday.
The action on either side of the border sparked concerns for those living in the South Korean islands nearby, as they were told to find safety in bomb shelters.
Residents in Yeonpyeong, last hit by North Korea in 2010, were told to stay inside.
But no reports suggested shells crossed into South Korean territory.
Kim denied North Korea’s shell firings on Saturday, instead dubbing the move a “deception tactic”.
South Korea’s military dubbed Kim’s statement psychological warfare, urging North Korea to stop any military activity causing tension near the border.
In December meetings, Kim Jong-un threatened a nuclear attack on the South and said an armed conflict could “break out at any time”.
And in Ukraine, Russia has used North Korean missiles to blitz the invaded country for the first time since Putin’s 2022 invasion.
The missile bombardments on December 29 and January 2 left at least 50 people dead.
While Kim has been tipped previously as a potential successor, Kim Jong-un’s 11-year-old daughter is most likely the next tyrant-in-line.
Kim Ju Ae was acknowledged on Friday for the first time as the despot’s heir by the National Intelligence Service.
But the NIS said it was still considering “all possibilities” in the succession plan.
“Based on a comprehensive analysis of public activities and the level of respect for Kim Ju Ae since her initial public appearance, at present, she appears to be the most likely successor,” said the NIS.
The intense speculations came after the young heiress was seen with her father in public outings last month.
Most recently, she accompanied him to the December launch of North Korea‘s Hwasong-18 solid-fuel intercontinental ballistic missile.