The statement from the country's top military body, the National Defence Commission (NDC), came after reported moves by Washington and Pyongyang to revive long-stalled six-nation talks on denuclearisation.
The NDC said recent comments from President Barack Obama had revealed that the United States' goal was to "bring down" North Korea.
The statement was an apparent reaction to an interview Obama gave on January 22 in which he spoke of the eventual collapse of the North Korean regime, calling it "the most isolated, the most sanctioned, the most cut-off nation on Earth".
"Since the gangster-like US imperialists are blaring that they will 'bring down' the DPRK (North Korea)... The army and people of the DPRK cannot but officially notify the Obama administration... That the DPRK has neither need nor willingness to sit at negotiating table with the US any longer," the NDC said.
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If the US ignites "a war of aggression" and unleashes a nuclear war, North Korea will "counter it through its own nuclear strikes", it said in a statement carried by the North's official Korean Central News Agency (KCNA).
The Washington Post reported on Monday that US and North Korean nuclear envoys had been secretly discussing the idea of "talks about talks", but had been unable to agree on practical arrangements.
US State Department spokeswoman Jen Psaki said in response that Washington's position had not changed and that it "continues to offer Pyongyang an improved bilateral relationship" provided it takes action on denuclearisation.