The National Intelligence Estimate (NIE) added that US charges about Tehran's atomic goals have been overblown for at least two years, and comes after US President George W. Bush raised the specters in recent months of "World War III" or a "nuclear holocaust" if Iran got an atomic arsenal.

"The report's conclusions justify the action already taken by the international community to get to the bottom of Iran's nuclear programme and to increase pressure on the regime to stop its enrichment and reprocessing activities," a British foreign ministry spokeswoman told AFP.

She added that Britain would be discussing the report in detail with American analysts in the coming days.

The NIE, the consensus view of all 16 US spy agencies, said it was unclear whether Iran sought nuclear arms and that halting its still-dormant programme suggested greater susceptibility to global pressure than had been thought.

It also cautioned that the Islamic republic was keeping its nuclear options open, still bucked international demands to freeze uranium enrichment, and that Tehran could have the technical ability to make a nuclear weapon sometime between 2010 and 2015.