Kim Yo Jong -- long one of her brother's closest advisers -- was named in January as a first vice-department director of the Central Committee of the ruling Workers' Party.

Her issuing a statement of direct political significance highlights her central role in the North's hierarchy.

Seoul's security ministers expressed "strong concern" after Pyongyang fired what the South's military said appeared to be two short-range ballistic missiles on Monday.

The North's state media published pictures Tuesday of Kim Jong Un supervising what it called a "long-range artillery" drill and followed up later with the statement from his sister.

It was "strange" to Pyongyang to hear the South's presidential office expressing strong regret and demanding an end to such actions, the official KCNA news agency cited her as saying.

"It is a presumptuous, silly action," she said.

"Such a nonsensical claim and speech of Cheong Wa Dae only amplifies our distrust, hatred and contempt for the entire south."

Kim Yo Jong 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.

She has frequently been pictured alongside him at summits with US President Donald Trump or the South's President Moon Jae-in, but has not previously been named as making a political pronouncement of her own.