The South's defence ministry stated Kim Yo-jong, the powerful sister of the North's leader Kim Jong-un, made the threat in a commentary carried by its state media, just a day after the reclusive state claimed the South had sent unmanned drones over Pyongyang three times this month, Yonhap news agency reported.
"We clearly warn that if North Korea inflicts harm on the safety of our people, that day will be the end of the North Korean regime," the defence ministry said in a statement released to reporters.
"Kim Yo-jong's remarks reflect the North's hypocritical behaviour, which continues with provocations, and has recently resorted to vulgar and petty tactics, like floating trash-filled balloons," it said.
In the statement, Kim said the North was ready to take a "strong corresponding retaliatory action" in case drones carrying anti-Pyongyang materials are flown again into the North, warning that the "attack time" can come at any time.
"That time is not set by us," Kim said. "The moment that a drone of the ROK is discovered in the sky over our capital city once again will certainly lead to a horrible disaster." ROK stands for South Korea's official name, the Republic of Korea.
The defence ministry's stance was echoed by the ruling People Power Party (PPP) on Sunday.
"South Korean security won't be shaken by the North's irrational and reckless provocations. The country has overwhelming military capabilities based on its alliance with the United States (in case of attacks from Pyongyang)," PPP spokesperson Han Zeea said in a statement.
On the same day, the North said it again floated trash-filled balloons across the border into the South in counteraction.
The South's defense ministry accused the North of shifting the blame to South Korean activists sending anti-Pyongyang leaflets, to make up for its "continued failure" in the running of state affairs, including the failed launches of military spy satellites.
The ministry called Kim's statement a "typical ploy" by the North to stoke an internal feud among South Koreans and it chose to do so again out of "anxiety" felt by the dictatorial regime.
"North Korean authorities should start off by stopping the internationally embarrassing trash balloon launches, rather than being so frightened by just one drop of 'drone leaflets' that are unidentifiable," it said.
The defence ministry also said the North has already intruded into the South's airspace "over 10 times," in an apparent reference to incidents dating back to 2022.
On Friday, North Korea's foreign ministry claimed that South Korean drones carrying leaflets were detected in the night skies over Pyongyang on October 3, as well as Wednesday and Thursday of this week, and threatened to respond with force if such flights occur again.
Defense Minister Kim Yong-Hyun initially denied that the military had sent any drones across the border, but the Joint Chiefs of Staff later said it could not confirm whether the North's claims were true.
In Sunday's statement, Kim Yo-jong took issue with the South Korean military's stance that it "cannot confirm" the North's claims over the drones, saying the military's response essentially admits that it is accountable.
"Such attitude of neither denying nor admitting the case proves that the military admitted by itself that it is the chief criminal or accomplice of the current incident as a revelation of its mental state that it can admit the case, unable to deny it," she said.
The North's state-run newspaper featured angry reactions from its citizens over the alleged flight of the drones on its front page Sunday, in an apparent effort to incite hostility toward the South.
The Rodong Sinmun reported that "millions of our people are boiling with unstoppable rage" and "roaring" with the desire for "merciless revenge."
The newspaper quoted some residents as calling the South "puppet trash," "scum" and "rats," and wanting to "quickly turn the enemy's stronghold into a sea of fire."
The newspaper also carried Kim's latest statement about the drones on the front page, a rare move considering that the North typically doesn't disclose its statements directed at the South or the US to its people.
The use of the alleged drone flights is seen as a propaganda manoeuvre by the reclusive regime to fuel animosity toward the South as a way to justify leader Kim's drive to drop the unification policy and define the two Koreas as states that are hostile to each other.