The US has signalled a major change in policy towards Iran, offering economic incentives for the Islamic state to give up its alleged nuclear ambitions.
The US had previously taken an uncompromising line on Iran
It will lift a decade-long block on Iran's membership of the World Trade Organization, and objections to Tehran obtaining parts for commercial planes.
The move will also see the US get behind European attempts to use diplomacy to resolve the issue.
A Tehran official said the US move was "too ridiculous to be called an offer".
Sirus Naseri, who is involved in negotiations with the Europeans, told CNN it was "like trading a lion for a mouse".
"Would the United States be prepared to give up its own nuclear fuel production against a cargo of pistachios delivered in truckloads?" he said.
Tehran denies seeking nuclear weapons and has suspended uranium enrichment.
The US and European Union want that move made permanent, and have threatened to seek United Nations sanctions if Iran does not comply.
There have been suggestions that the US is planning to use military force against Iran's suspected nuclear weapons facilities.
US President George W Bush said on Friday: "I'm pleased that we are speaking with one voice with our European friends.
"I look forward to working with our European friends to make it abundantly clear to the Iranian regime that the free world will not tolerate them having a nuclear weapon."
Two of the most senior figures in the US administration made it clear that the US now backed European-led diplomatic efforts on Iran, but said the possibility of punitive action remained.
"The decision that the president has taken is that the United States will make an effort to actively support the EU3 negotiations with the Iranians," Secretary of State Condoleezza Rice said, referring to the EU's talks with Iran led by Britain, France and Germany.
EU nations have been involved in negotiations with Iran
"We will make clear that we will lift our objections to an Iranian application to the WTO, and that we are prepared to lift an objection to the licensing of spare parts for Iranian commercial aircraft," she added.
But Vice-President Dick Cheney sounded a note of caution.
"At the end of the day, if the Iranians don't live up to their obligations and their international commitments to forego a nuclear programme, then obviously we'll have to take stronger action," he told Fox News.
Previously, the US had refused to offer incentives for Iran to abide by the terms of the Nuclear Non-Proliferation Treaty.
BBC state department correspondent Jonathan Beale says Mr Bush appears to have had a change of heart following his recent visit to Europe and discussions with EU leaders.
Ms Rice insisted: "This is most assuredly giving the Europeans a stronger hand, not rewarding the Iranians."
Iran insists it wants nuclear power only for peaceful purposes.
Tehran has agreed to maintain its enrichment suspension while it negotiates trade and security benefits, but maintains that the enrichment issue is not currently up for discussion.
Earlier, the "EU3" said progress on nuclear talks with Iran had not been "as fast as we would wish".