Donald Trump has warned Kim Jong Un that he has a nuclear button which is "much bigger" than that of the North Korean leader.
In a tweet on Wednesday, the US President responded to a speech made by the North Korea leader in which he said "a nuclear button is always on my desk".
It comes as a key communication line between North and South Korea is reopened.
Mr Trump wrote: "North Korean leader Kim Jong Un just stated that the 'nuclear button is on his desk at all times'.
"Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works!"
The President's tweet - his 17th of the year - came after the US ambassador to the United Nations, Nikki Haley, warned Pyongyang against staging any further missile tests.
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She told reporters that the US was hearing reports that the rogue state might be preparing another launch.
"I hope that doesn't happen. But if it does, we must bring even tougher measures to bear against the North Korean regime," she said.
Mr Kim has reopened a key cross-border channel with South Korea for the first time in nearly two years after saying he was open to dialogue with his neighbours.
Mr Kim said during his New Year's Day speech that he is considering the possibility of sending athletes to next month's Winter Olympics in Pyeongchang, with South Korea offering to hold talks next week.
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But despite the olive branch, Mr Kim said his country would push ahead with "mass producing" nuclear warheads in defiance of UN sanctions.
Ms Haley added that Washington would not take any talks between North and South Korea seriously if Mr Kim did not give up his nuclear weapons.