Iran has indicated it may suspend some of its nuclear activities to end a stand-off over its nuclear programme.
Iran denies it wants to build nuclear weapons
"We are trying to choose the best course of work," Iran's top nuclear negotiator told state television.
Iran had described as "unbalanced" a European offer of trade concessions and nuclear technology in return for the suspension of uranium enrichment.
The UN's nuclear agency has given Iran until the end of November to suspend its enrichment programme.
Iranian negotiator Hassan Rowhani said that an indefinite suspension did not mean a permanent suspension.
"The European proposal for an unlimited suspension of uranium enrichment can be implemented, provided it does not contradict the Islamic republic's criteria," he said on state television, according to the AFP news agency.
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.
Iran suspended enrichment a year ago as a confidence-building measure, but has continued activities such as building the centrifuges that refine the uranium.
Closed-door talks between Britain, Germany, France and Iran took place earlier this week in Vienna, in what an Iranian spokesman described as a good atmosphere.
Foreign ministry spokesman Hamid Reza Assefi said negotiations were continuing and would resume on Wednesday with Iran putting forward counterproposals.
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 UN's International Atomic Energy Agency have turned up no hard evidence of an Iranian weapons programme.