Iran's chief nuclear negotiator says he feels positive about an incentives package agreed by world powers on Tehran's nuclear programme.
Ali Larijani is in Europe for talks on the offer
But Ali Larijani said Iran should not be given a deadline for its response to the offer, which is aimed at suspending Tehran's uranium enrichment work.
Mr Larijani is in Europe for talks on the package. Western nations have indicated they want a reply by 15 July.
Earlier, the EU said talks with Iran on the nuclear issue had begun well.
Iran says its nuclear programme is entirely peaceful, aimed at meeting its energy needs. But the US and other Western nations fear Iran may develop nuclear weapons.
Mr Larijani was speaking after meeting Spanish government officials in Madrid.
On Thursday he held talks with the European Union's foreign policy chief, Javier Solana, in Brussels, which the EU described as a "good start".
Mr Larijani said in Madrid: "We view this proposition in a positive light but there are doubts about it which we have to clarify through dialogue... I see no reason to be pessimistic."
However, he added: "We do not need to be given a time limit to respond... that does not make sense. If negotiation is to take place it needs to be in the language of negotiation."
Western powers had said they wanted a reply to the package - believed to include a nuclear reactor and fuel supply along with trade and other incentives - before a G8 summit in St Petersburg on 15 July.
The deal was agreed by the five UN Security Council nations plus Germany.
The US has threatened to push for sanctions if there is no positive response from Iran.
UN nuclear watchdog chief Mohamed ElBaradei on Wednesday warned that the world was "impatient" at Iran's failure so far to respond.
Iranian President Mahmoud Ahmadinejad had earlier said a formal response could come in August.