Kim Yo Jong, the younger sibling of the North Korean leader, is in charge of propaganda affairs for the country and often appears at her brother's major public events - including summits with President Donald Trump and other regional leaders.
But her statement carried by state media was the first of its kind, indicating a further elevation of her political status and rising stock among North Korea's ruling class.
In the statement issued on Tuesday night, she criticised South Korea's presidential Blue House for expressing strong concerns over the North's firing drills and urging it to stop acts that do not help reduce military animosities.
'As far as I know, the South side is also fond of joint military exercises and it is preoccupied with all the disgusting acts like purchasing ultra-modern military hardware,' Ms Kim said.
Kim Yo Jong, the younger sibling of the North Korean leader, is in charge of propaganda affairs for the country and often appears at her brother's major public events - including summits with President Donald Trump and other regional leaders
Kim Yo Jong, North Korean leader Kim Jong Un's sister, talks with South Koran President Moon Jae-in as they watch a performance of North Korea's Samjiyon Orchestra at National Theater in Seoul, February 2018
'They meant they need to get militarily prepared but we should be discouraged from military exercises.
'Such a gangster-like assertion can never be expected from those with normal way of thinking.'
Describing the Blue House as 'a mere child' and 'a burnt child dreading fire', she questioned how its words and actions could be 'so perfectly foolish in detail'.
Ms Kim did not name liberal South Korean President Moon Jae-in, whom she has met several times.
She only said: 'The South side's response is so regretful and disappointing but it is somewhat fortunate that it was not direct statement of the president.'
South Korea's Unification Ministry, which handles inter-Korean affairs, said it has no specific comment on Ms Kim's statement.
But spokesman Yoh Sang-key said the two Koreas should maintain mutual respect while working toward establishing a peace on the Korean Peninsula.
Earlier Tuesday, state media said leader Mr Kim supervised a live-fire rocket artillery exercise in an apparent reference to the two suspected short-range ballistic missile launches detected by South Korea's military a day earlier.
Kim Yo Jong, left, sister of North Korean Leader, walks ahead of South Korean President Moon Jae-in and North Koran leader Kim Jong Un, right, as they arrive at the headquarters of the Central Committee of the Workers' Party in Pyongyang, North Korea, September 2018
'As far as I know, the South side is also fond of joint military exercises and it is preoccupied with all the disgusting acts like purchasing ultra-modern military hardware,' Ms Kim said
On Saturday, North Korea said Mr Kim also guided an artillery drill aimed at testing the combat readiness of military units.
The back-to-back firing exercises were an apparent show of force by Mr Kim, who had earlier vowed to bolster his nuclear deterrent and warned of 'shocking action' over now-stalled nuclear negotiations with Mr Trump.
The latest firing drills were his first weapons tests since late November.
Kim Yo Jong's statement was issued in her capacity as a first vice-department director of the Workers Party's Central Committee.
She also serves as an alternate member of the North's powerful Politburo and a member of the rubber-stamp parliament.
South Korean officials and experts say she is virtually the North's top propaganda official.