Iran has described as "unbalanced" a European offer of trade concessions and nuclear technology in return for the suspension of uranium enrichment.
Iran denies it wants to build nuclear weapons
But an Iranian spokesman said Tehran would continue to discuss the proposal made by Britain, France and Germany.
The UN's nuclear agency, the IAEA, has given Iran until the end of November to suspend its enrichment programme.
Iran says the programme is purely for peaceful purposes, but the US accuses it of developing nuclear weapons.
Enriched uranium can be used for weapons as well as fuel.
"The European proposal is their preliminary proposition and is not definitive, but it is unbalanced," Foreign Ministry spokesman Hamid Reza Assefi said.
He said negotiations were continuing and would resume on Wednesday with Iran putting forward counterproposals.
Mr Assefi said the Europeans had not asked for a lasting and unlimited suspension of enrichment, but if they did it would be out of the question.
The BBC's Frances Harrison in Teheran says it is hard to see how a compromise can be reached without addressing the enrichment issue once and for all, but it may be a question of finding the right wording to allow Iranian negotiators to sell the deal to their own people.
The European proposal asks Iran to suspend its uranium enrichment activities in return for incentives and alternative nuclear technology such as a light-water reactor, according to leaked reports.
Closed-door talks between the three European countries and Iran, took place earlier this week in Vienna, in what an Iranian spokesman described as a good atmosphere.
European diplomats have said that if Iran rejects the deal, most EU countries would back US proposals for the UN Security Council to impose economic sanctions on Iran.
So far, two years of investigation by the IAEA have turned up no hard evidence of an Iranian weapons programme.