Iran's supreme leader Ayatollah Ali Khamenei says he would rather see no deal on the country's nuclear programme than one that undercuts Iranian interests.
He would back an accord, but only if neither side got everything it wanted, he said in a statement.
The comments came as the US and Iran held further talks in Munich.
Iran and six international powers are aiming to reach a framework agreement in March and a final deal by 30 June.
The world powers want Iran to curb its nuclear programme in return for the lifting of sanctions.
Ayatollah Khamenei said he backed President Hassan Rouhani's line that "negotiations are all about trying to reach a common stand".
This meant that no side could end up getting everything it wanted, he said, warning that "it's better to have no deal than one that goes against our national interests".
He also expressed reservations about the current plan of reaching an agreement in two stages, first on general issues and then on details.
"We don't approve of this as we don't trust the other side. They would use the general agreement to put pressure on us on details. Any agreement should be reached in one stage," he said.
Iran would consider the deal unfavourable if sanctions were not lifted with immediate effect after a comprehensive accord, Bozorgmehr Sharafedin from the BBC's Persian service reports.
Currently, the final agreement aimed for by the end of June foresees a phased lifting of sanctions in return for Iran curbing its nuclear ambitions.
The statement by Iran's supreme leader came as Iran's Foreign Minister Mohammad Javad Zarif met US Secretary of State John Kerry for a second time on the sidelines of the international security conference in Munich.
"We need to seize this opportunity," Mr Zarif said afterwards, adding that he did not think a further extension to talks would be productive.
"We are reaching the point where it is quite possible to make an agreement and I do not believe anything will be different in a year down the road."
The talks had previously been due to end in November 2014, but were extended to June 2015 after the parties failed to reach a comprehensive agreement.
The West suspects Iran is covertly seeking nuclear weapons capability. Iran denies it has a nuclear arms agenda and insists it is using uranium enrichment purely for energy purposes.
The dispute which has been going on for over a decade has left Iran isolated and suffering from international economic sanctions.
Mr Rouhani, who took office in 2013, has tried to make a fresh start in negotiations but is coming under pressure from hardliners in parliament.
Iran plus its six international interlocutors - the US, Britain, France, Germany, Russia and China - are currently working on the basis of an interim agreement which allows them until 30 June to reach a final accord.