In a speech to elderly veterans of the Korean War, Kim vowed to "annihilate" South Korea if Seoul ever tries to launch a pre-emptive strike.
Marking the 69th anniversary of the armistice that brought the conflict to an end, he said: “I once again make it clear that North Korea is fully ready for any military confrontation with the United States.
“Our armed forces are thoroughly prepared to respond to any crisis, and our nation’s nuclear war deterrent is also fully ready to mobilise its absolute strength faithfully, accurately and promptly to its mission.”
Kim Jong-un speaking in North Korea KCNA
Kim Jong-un greeting veterans of the Korean War KCNA
It comes after weeks of warnings from Washington and Seoul that Pyongyang has finalised preparations for another nuclear test.
In his speech, Kim picked out new South Korean president Yoon Suk-yeol – not least because Yoon is taking a much harder line on relations with the North than his predecessors.
Kim said: “There is no way to [achieve peace] if the South Korean government and military thugs come up with ways to seek military confrontation and believe that they can pre-emptively incapacitate and smash part of our military power with specific military ways and means.
“Such dangerous attempts will be immediately punished with powerful force and the Yoon Suk-yeol government and its military will be annihilated.”
The North has held 18 separate rounds of missile and rocket tests this year.
The test site at Punggye-ri is ready for what would be Pyongyang’s seventh nuclear test, the International Atomic Energy Agency has confirmed.
If Kim does go ahead with another test, it will no doubt provoke an international security crisis.