A US spokesman repeatedly referred to 33-year-old North Korean leader Kim Jong-Un's youth Wednesday while warning the "young dictator" against further nuclear provocations.

Kim announced Wednesday that his isolated pariah state had successfully miniaturized a thermo-nuclear warhead, an unproven claim but one that will further inflame tensions.

Pyongyang carried out a nuclear weapons test in January and a banned missile launch last month, defying threats of tighter international sanctions.

State Department spokesman John Kirby issued the now traditional slap down of Kim's "provocative rhetoric" -- but with a new twist, which was mockery of his age.

"I'd say that the young man needs to pay more attention to the North Korean people and taking care of them than in pursuing these sorts of reckless capabilities," said Kirby.

Kirby, a retired admiral who was already commissioned as a US navy officer when Kim was only three years old, could not confirm the claim, but said "we take these comments seriously."

"We have to. I mean, this young man has proven that he is perfectly willing and able to flout and to violate his international obligations," he added.

Challenged on his repeated use of the term, Kirby said: "Factually, it's true. He's young and he's a man." Then he returned to his theme.

"But look, clearly, this young dictator continues to violate international obligations and continues to ignore the desperate needs of his own people and continues to increase the tensions of the peninsula rather than work to decrease the tensions," he said.

While it may be impossible to know whether the State Department's tactic will succeed in getting under the young leader's skin, North Korea does like to employ its own rhetorical flourishes.

Threatening to launch a pre-emptive nuclear strike this week, state news agency KCNA warned: "If we push the buttons to annihilate the enemies even right now, all bases of provocations will be reduced to seas of flames and ashes in a moment."