"The fact that they have a warhead that's fitable to the Rodong (ballistic missile) is pretty much given," said Arthur Brown, who until 2005 was National Intelligence Officer for East Asia at the US Central Intelligence Agency (CIA).

"We (the United States) went from nothing to missile capable in seven years. The Russians went from their first test to missile capable in six... Why do we think the North Koreans can't have that kind of technology?"

What was unclear was how much uranium and, or, plutonium North Korea possessed and how much they need for each of their weapons, he told a press conference at the Foreign Correspondents' Club of Japan.

"If we knew these two numbers then we could run the maths and say how many weapons they actually have," said Brown, who did not disclose his sources and said it was tough to get accurate information.

North Korea, which tested a nuclear weapon for the first time two years ago, said this week it would start work to resume plutonium reprocessing at its Yongbyon complex, possibly within a week.

The standoff comes amid reports that North Korean leader Kim Jong-Il suffered a stroke around mid-August.

"It was a serious stroke," said Brown. "It's not a light stroke. He has recovered a bit but I think no one expects him to come back into a full recovery mode at this stage."

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