A North Korean foreign ministry spokesman on Monday described Bush as "imbecile" and a "tyrant" as he reacted to the US president calling North Korean leader Kim Jong-Il a tyrant during election campaigning last week.

The North Korean spokesman also termed Bush worse than Adolf Hitler and warned that hostile US policy would make it impossible for Pyongyang to attend six-nation working-level talks on the nuclear issue, which had been scheduled for August but have yet to materialise.

The US State Department rejected the attacks on Bush.

"I don't think it really merits a comment. Personal attacks on the president, obviously we reject them and they're obviously inappropriate, but there's really not much more to say," said Adam Ereli, the department's deputy spokesman.

He said he believed North Korea would attend, as agreed at the last round, the next working group and plenary meetings before the end of September.

"That agreement is still operative. We're still working with China and the other parties to schedule something, including a working group to precede the plenary. Those discussions are ongoing," he added.

Aside from the United States and North Korea, the talks bring together South Korea, China, Japan and Russia and have been going on for about a year without much success to end North Korea's nuclear weapons programs.

Bush, who had previously labeled North Korea as part of an "axil of evil," said during a campaign speech last Wednesday in Wisconsin that he had helped to organize the six party talks to pressure Pyongyang to end its nuclear ambition.

"So there's now five countries saying to the tyrant, disarm, disarm," he said.

Pyongyang in turn accused Bush of trying to topple the legitimate government of North Korea, branding him "a tyrant that puts Hitler into the shade" as well as "an idiot, an ignorant, a tyrant and a man-killer."

Ralph Cossa, president of the Pacific Forum of the US-based Center for Strategic and International Studies, said it was ironic that the State Department was calling North Korea's personal attacks inappropriate.

"Well, it would have been better if Mr Bush hadn't used the word 'tyrant' either ... the North Koreans don't make a distinction between calling someone a tyrant since to them Kim Jong-Il is God," he said.

"Hopefully, Mr Bush will understand that he is still president of the United States and that while this plays well to the good old boys at the Seven-Eleven, he has to sort of keep an eye on the problems he is causing for his own diplomats," Cossa said.

North Korea's angry response came as US and South Korean troops Monday began 12 days of annual military exercises to test their response to a possible invasion by North Korea.

The North insists the exercise involving 14,500 troops is part of Washington's war preparations to topple the Stalinist regime.

The stand-off over the North's quest for nuclear weapons erupted in October 2002 when Washington accused Pyongyang of operating a secret nuclear program based on enriched uranium in violation of a 1994 freeze of its separate plutonium producing program.

Pyongyang has denied running the uranium-based program, but has again fired up its once-mothballed nuclear reactor to extract plutonium. The CIA estimates it has at least one or two nuclear bombs.

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