The Democrats in the United States have called for a rethink of policy on Iran.
Iran insists uranium enrichment is for peaceful purposes only
The call comes after a major new intelligence assessment suggested the government in Tehran is not trying to develop nuclear weapons at present.
The latest National Intelligence Estimate says it is now believed Iran stopped its weapons programme in 2003.
But Israeli Prime Minister Ehud Olmert said it was vital to continue US-led efforts to prevent Iran from obtaining a nuclear weapon.
Earlier his Defence Minister, Ehud Barak, cast doubt on the report's findings, saying that while Iran might have stopped its suspected weapons programme it had probably restarted it again.
Reports of this kind were "made in an environment of high uncertainty", he added.
The Democrat leader of the US Senate, Harry Reid, said he hoped the White House would undertake "a diplomatic surge" to engage with Iran.
Stephen Hadley said Bush's strategy was right
"I hope this administration reads this report carefully and appropriately adjusts its rhetoric and policy vis-a-vis Iran," said Mr Reid.
He added that the Bush administration should emulate former President Ronald Reagan's engagement with the Soviet Union.
The BBC Washington correspondent Justin Webb says the Democrat response is a sign of the pivotal importance the new intelligence assessment might turn out to have.
But, our correspondent adds, Washington's Iran hawks are horrified.
In particular 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.
A senior adviser to President Bush said the report was "positive" but the risk of a nuclear Iran remained "serious".
"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.
Iran is currently under sanctions from both the UN Security Council, which is demanding the end of uranium enrichment, and unilateral US sanctions.
The declassified summary of the report, which draws together information from the US's 16 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 - a process necessary for producing the fissile nuclear material needed to build a nuclear bomb, the report says.
But the report's authors judge with "moderate confidence" that Iran "still faces significant technical problems" operating the new equipment.
And they conclude that the country is not likely to have enough highly enriched uranium to build a bomb until 2010-2015.
US National Security Advisor 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 international community should "turn up the pressure on Iran" using diplomatic isolation, UN sanctions and other financial leverage, he said.
The BBC News website's world affairs correspondent, Paul Reynolds, says the report is cautious in its assessment of Iran's nuclear activities and provides little evidence for those who would like an early military attack.
He says it will strengthen the hand of those who want further sanctions since it states that past pressure has worked.