A US National Intelligence Estimate agreed to by all 16 US spy agencies cited "high confidence" that Tehran halted its nuclear weapons program in late 2003 and "moderate confidence" that it had not restarted as of mid-2007.

At the same time, the Islamic republic is thought to be "keeping open the option to develop nuclear weapons," according to declassified key findings of the report, which is based on intelligence available as of October 31.

"But we do not know whether it currently intends to develop nuclear weapons," according to the estimate.

"Tehran's decision to halt its nuclear weapons program suggests it is less determined to develop nuclear weapons than we have been judging since 2005," it said, adding Iran appeared to have bowed to international pressure.

Over the past two years, US President George W. Bush and other top US officials have accused Tehran of seeking nuclear weapons and rallied global pressure on Iran to freeze uranium enrichment and reprocessing activities.

"Our assessment that the program probably was halted primarily in response to international pressure suggests Iran may be more vulnerable to influence on the issue than we judged previously," the NIE said.

National Security Adviser Stephen Hadley said in a statement the NIE suggested that the US strategy of ensuring "that the world will never have to face a nuclear-armed Iran" was on the right track.

"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.

Hadley said the finding "offers some positive news."

"It confirms that we were right to be worried about Iran seeking to develop nuclear weapons. It tells us that we have made progress in trying to ensure that this does not happen," he said.

"But the intelligence also tells us that the risk of Iran acquiring a nuclear weapon remains a very serious problem.

"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."