North Korea has described US President George W Bush as an "imbecile" and a "tyrant that puts Hitler in the shade".
The US wants to shut down all North Korea's nuclear facilities
A Foreign Ministry spokesman was responding to comments President Bush made last week in which he described the North's Kim Jong-il as a "tyrant".
The spokesman also reiterated that North Korea will not attend a working meeting ahead of the next round of six-party talks on its nuclear programme.
The working group is due to meet later this month in New York.
President Bush explained in a speech in Hudson, Wisconsin, last Wednesday, his decision to ask other countries in the region to help him persuade the North to disarm.
"I felt it was important to bring other countries into the mix, like China and Japan and South Korea and Russia, so there's now five countries saying to the tyrant in North Korea, disarm, disarm," he said.
A North Korean Foreign Ministry spokesman, in comments carried by state news agency KCNA, responded: "This clearly proves that the DPRK [North Korea] was right when it commented that he is a political imbecile bereft of even elementary morality.....
"Bush is a tyrant that puts Hitler into the shade and his group of such tyrants is a typical gang of political gangsters," he said.
North Korea reiterated comments it made last week that it could not now take part in
working-level talks ahead of six-party discussions on its nuclear programme scheduled to take place by the end of September.
The spokesman said this was because "the US has become more undisguised in pursuing its hostile policy towards the DPRK, backtracking from all agreements and common understanding reached at the third round of the six-party talks" [held in June].
At those latest talks, the US proposed North Korea freeze
its nuclear programme as a step towards eventual dismantlement and consequent economic rewards.
But Pyongyang has questioned the timeframe, and also continues to deny US claims that it has a second enriched uranium weapons programme.
The Foreign Ministry spokesman said on Monday that instead North Korea would boost a "thousand times" its capacity for self-defence.
The nuclear dispute flared up in 2002, when US officials accused North Korea of running a secret nuclear programme in violation of international agreements.
Since then there have been a series of talks in an effort to resolve the crisis, but a deal has yet to be reached.