Iran has welcomed a major US intelligence report that suggests its government is not currently trying to develop nuclear weapons.
Iran says it will continue uranium enrichment despite sanctions
The latest National Intelligence Estimate says it is now believed Iran stopped its weapons programme in 2003.
Tehran has always maintained its nuclear programme is being developed purely for peaceful purposes.
But the US and other Western powers say Iran is trying to build a nuclear weapons capability.
Iran is currently under UN Security Council and unilateral US sanctions.
But the BBC News website's world affairs correspondent, Paul Reynolds, says the question of sanctions remains active because Iran is still defying Security Council calls for it to suspend uranium enrichment.
The confrontation is now likely to continue indefinitely but at a lower temperature, he says.
Iranian Foreign Minister Manouchehr Mottaki said he welcomed the change of opinion.
"It's natural that we welcome it when those countries who in the past have questions and ambiguities about this case... now amend their views realistically," he said.
Iranian state TV hailed the report as a "victory". It said Iran was "honest" and had been "vindicated", while it said the report demonstrated flaws in US intelligence.
The head of the International Atomic Energy Agency, Mohamed ElBaradei, said the new intelligence "should help to defuse the current crisis".
But he said that although the agency had no evidence of an ongoing Iranian nuclear weapons programme, Iran should still clarify some aspects of its past and present nuclear activities.
Earlier US Democrats called for a major policy rethink in the light of the NIE report.
However, Israeli Prime Minister Ehud Olmert said it was vital to continue US-led efforts to prevent Iran from obtaining a nuclear weapon.
Israeli Defence Minister Ehud Barak took issue with the report, saying Iran had probably restarted its suspected weapons programme.
"It seems Iran in 2003 halted for a certain period of time its military nuclear programme but as far as we know it has probably since renewed it," he told Israeli radio.
Reports of this kind were "made in an environment of high uncertainty", he added.
The BBC's Justin Webb in Washington says US hawks are horrified by the report.
They are concerned that news of that key assessment will drown out other findings in the document - for instance, that Tehran is keeping open its option to develop a nuclear weapon and would have all the capacity to do so in the future if its leaders decided to go ahead.
US National Security Adviser Stephen Hadley said the report's findings confirmed the US was "right to be worried" about Iran's nuclear ambitions and that President George W Bush had "the right strategy".
The declassified summary of the report, which draws together information from the 16 US intelligence agencies, says with "high confidence" that Iran stopped its nuclear weapons programme in 2003 "in response to international pressure".
The assessment says with "moderate confidence" that the programme has not restarted.
This is a turnaround from previous assessments, when US intelligence agencies believed Iran was trying to develop a nuclear weapon.
Iran made "significant progress" in 2007 installing gas centrifuges used to enrich uranium but still faced technical problems operating them, the report said.
It concludes that the country is not likely to have enough highly enriched uranium to build a bomb until 2010-2015.