North Korean leader Kim Jong Un's sister condemned Seoul as a "frightened dog barking" on Tuesday over its response to a weapons drill by Pyongyang, state media reported.
Kim Yo Jong's comments -- the first time she has issued a political statement in her own name -- came after Seoul's security ministers expressed "strong concern" following Pyongyang's firing of what the South's military said appeared to be two short-range ballistic missiles on Monday.
North Korean state media published pictures Tuesday of Kim Jong Un supervising what it called a "long-range artillery" exercise.
Seoul said it was the North's first weapons test for more than three months. The North is banned from testing ballistic missiles under UN Security Council resolutions.
The South Korean military said two devices were fired eastwards over the sea from the Wonsan area on the North's east coast and flew 240 kilometres (150 miles) at a maximum altitude of 35 kilometres.
The North also appeared to be carrying on a firing drill that began Friday, it added.
Kim Yo Jong said the nuclear-armed North was entitled to carry out military drills and the South's condemnation was a "truly senseless act" and "perfectly foolish", according to a statement published by the North's official Korean Central News Agency.
She likened it to "a frightened dog barking".
"Such incoherent assertion and actions made by Chongwadae only magnify our distrust, hatred and scorn for the south side as a whole," she added, referring to the South's presidential office.
Kim Yo Jong has long been one of her brother's closest advisers and has frequently been pictured alongside him at summits with US President Donald Trump or the South's President Moon Jae-in.
She acted as her brother's envoy to the South at the time of the Pyeongchang Winter Olympics in 2018, which ushered in a rapid diplomatic rapprochement on the divided peninsula.
The issuance of a political statement in her own name highlights her central role in the regime, analysts said.
"By having someone like Kim Yo Jong -- who has been a dovish figure towards the South and acting as a soft and friendly messenger -- issue such a strong statement condemning the current government, the North is raising its pressure," said Go Myong-hyun, an analyst at the Asan Institute for Policy Studies.
While she was named in January as a first vice-department director of the Central Committee of the ruling Workers' Party, the statement signalled she may have moved further up the hierarchy, he added.
If she went on to issue direct condemnations of the US, he told AFP , it would be confirmation that she had become "the second most powerful person after her brother".
- Deadlock -
Monday's drills came days after the one-year anniversary of the failed Hanoi summit between Trump and Kim Jong Un, and followed the expiry of an end-of-year deadline Pyongyang set Washington to offer it fresh concessions.
The two countries are deadlocked over sanctions relief and what the North would be willing to give up in return, while there are no significant contacts between the North and South.
Kim Jong Un has declared the North no longer considered itself bound by its moratoriums on nuclear and intercontinental ballistic missile tests, and threatened to demonstrate a "new strategic weapon" soon.
Seoul and Washington last week postponed forthcoming joint military exercises over the coronavirus outbreak -- the South has more than 5,000 infections -- and the South's President Moon offered to co-operate over the disease with the North, which has closed its borders to try to prevent a potentially devastating outbreak.
The moves had generated "no goodwill" from the North, said Leif-Eric Easley, a professor at Ewha University in Seoul.
Pyongyang was making clear "it will continue to improve military capabilities and make outsized demands, despite the political and public health preoccupations of Beijing, Seoul and Washington", he said.
North Korean leader Kim Jong Un did not don a facemask to supervise the firing of what state media called a 'long-range artillery' drill