Iran has rejected the latest European proposals for resolving concerns over the country's nuclear programme.
Iran has threatened to resume work at its Isfahan nuclear plant
The US had backed the package, which includes a proposal to let Iran develop a civilian nuclear programme if it halts uranium enrichment activities.
Tehran has threatened to resume nuclear activities next week that have been suspended since November 2004.
Its rejection of the EU proposals came as the new hardline President, Mahmoud Ahmadinejad, was sworn in.
Iran says its nuclear programme is peaceful, but Western countries suspect it is a front hiding efforts to build atomic bombs.
The European Union - through Britain, Germany and France - has been trying to find a compromise solution over Iran's nuclear plans for two years.
But correspondents say the latest proposals fail to contain any fundamental new concessions.
"The Europeans' submitted proposals regarding the nuclear case are not acceptable for Iran," foreign ministry spokesman Hamid Reza Asefi was quoted as saying.
"The proposals are unacceptable because Iran's right to enrich uranium is not included," Mr Asefi said.
He added that a full response to the EU's proposals would come on Saturday or Sunday.
Earlier, Under-Secretary of State Nicholas Burns said the US hoped Iran would take the proposal seriously and that Washington was "very much in support" of efforts by the three European states.
BBC State Department correspondent Jonathan Beale said the US move was highly significant, given that until recently, the US opposed Iran having its own civilian nuclear programme, as it suspected Tehran of wanting to develop nuclear weapons.
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 guarantees 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.
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.
Britain, France and Germany 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.