The US says it backs a European proposal to allow Iran to develop a civilian nuclear programme if it stops its uranium enrichment activities.
Iran has threatened to resume work at its Isfahan nuclear plant
Under-Secretary of State Nicholas Burns said the US hoped Iran would take the proposal seriously.
He also said he hoped Iran would not carry out its threat to resume nuclear activities next week, which have been suspended since last November.
Iran is due to respond to the offer on Sunday, but is expected to reject it.
A BBC correspondent in Washington says the US government's move on the Iranian nuclear issue is highly significant.
Until recently, the US opposed Iran having its own civilian nuclear programme - suspecting Tehran of wanting to develop nuclear weapons.
Nicholas Burns said on Friday Washington was "very much in support" of efforts by the three European nations - the UK, France and Germany - who have been negotiating with Tehran.
The EU plan - which has not been made public - is said to offer recognition of Iran's right to produce nuclear power for civilian purposes, as well improved trade relations with the EU, and guarantee of alternative nuclear fuel sources from Europe and Russia.
In return, the Europeans reportedly insist that Tehran should permanently give up nuclear enrichment and construction of a heavy-water reactor, which could be used to make a bomb.
"We think this proposal is a good one for the Iranians to consider and we would urge that they do so," said Nicholas Burns.
Iranian nuclear negotiator Hossein Mousavian told Reuters news agency Tehran would review the proposal and "definitely give our answer by Sunday".
But he was then quoted by the AFP news agency as saying the "the proposals are unacceptable" and a "clear violation" of agreements between Iran and the EU.
"They negate Iran's inalienable right (under the nuclear Non-Proliferation Treaty)," to making nuclear fuel, he was quoted as saying.
The BBC's Frances Harrison in Tehran says there is no new incentive on the table for Iran, and it is highly unlikely the proposals will be accepted.
Mr Mousavian also confirmed Iran intended to resume its uranium enrichment activities, which were suspended last November following international pressure.
Last week, Tehran said work at the uranium conversion plant near Isfahan would start again on Wednesday, and cited lack of progress in talks with the UK, France and Germany.
The three EU countries have called an emergency meeting of the UN's nuclear agency, the IAEA, on Tuesday.
The agency could refer the matter to the UN Security Council. Iran says its nuclear programme is peaceful, but Western countries suspect its programme is a front hiding efforts to build atomic bombs.