the White House has urged global powers to "turn up the pressure" on Iran.
It gave late 2009 as the "the earliest possible date" but added "that this is very unlikely".
we do not know whether it currently intends to develop nuclear weapons".
The report, based on intelligence up to October 31, also found that the Islamic republic is "keeping open the option to develop nuclear weapons", but admitted "
The consensus view of 16 US intelligence bodies - the National Intelligence Board.
Group chaired by by Director of National Intelligence, currently Michael McConnell.
Requested by senior civilian and military policymakers and politicians.
Normally remain classified.
2002 estimate on Iraqi weapons widely criticised as misleading.
Weeks after George Bush, the US president, warned of "World War III" or a "nuclear holocaust" if Iran got nuclear weapons, the NIE cited "high confidence" that Tehran had halted its nuclear weapons programme in late 2003 and "moderate confidence" that it had not restarted as of mid-2007.
Iran appears "less determined to develop nuclear weapons" than the US government has been claiming for the past two years, the NIE report said, and Tehran may be more susceptible to global pressure than the US previously thought.
White House angle
Despite the report,
"The intelligence ... tells us that the risk of Iran acquiring a nuclear weapon remains a very serious problem," Stephen Hadley, the national security adviser, said on Monday.
"America says Iran getting a nuclear weapon could cause WW3 but I think it's more likely that America would cause WW3 by invading Iran"
Jack, Bangor, UK
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"The bottom line is this: for that strategy to succeed, the international community has to turn up the pressure on Iran - with diplomatic isolation, United Nations sanctions, and with other financial pressure - and Iran has to decide it wants to negotiate a solution," he said.
"The estimate offers grounds for hope that the problem can be solved diplomatically - without the use of force - as the administration has been trying to do," he added.
Britain also said it favoured increasing the pressure on Iran over its nuclear programme despite the latest NIE report.
"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 said on Monday.
Iran denies Western charges that it seeks nuclear weapons under cover of a civilian atomic energy programme, and has drawn UN sanctions for refusing to freeze its uranium enrichment, which can yield materials for a nuclear bomb.
Mahmoud Ahmadinejad, Iran's president, said the issue of Tehran's controversial nuclear programme was "closed" and that his country was prepared for any eventuality.
"The nuclear issue is now closed. We do not feel threatened at all and we are prepared for any eventuality or conditions," he said during an annual summit of the Gulf Co-operation Council (GCC) in Qatar's capital, Doha.