Reports of a preliminary deal between Iran, the UK, France and Germany over Iran's nuclear programme have been given a cautious welcome.
Iran faces the threat of sanctions if it does not halt its nuclear plans
The head of the UN's nuclear watchdog said the deal was very tentative but he hoped it would lead to the suspension of uranium enrichment in Iran.
Javier Solana, foreign policy chief for the European Union, said an agreement with Iran seemed "pretty close".
Diplomats have said sticking points may be over the scope of the suspension.
Details of the accord reached in Paris at the weekend have not been released.
Hossein Mousavian, head of the Iranian delegation in Paris, earlier said fundamental principles had been agreed in the talks.
Europe wants Iran to suspend enrichment of uranium in return for a trade deal, and an opportunity to buy nuclear fuel.
Javier Solana told Reuters news agency: "Progress has been made and we are now waiting for the final response from Iran.
"It's very difficult to give a definition of how close we are but my feeling today is that we are pretty close to having an agreement... let's hope for developments in the coming hours, days."
Mohamed ElBaradei, head of the UN's International Atomic Energy Agency (IAEA), called the agreement "a step in the right direction".
A Western diplomat told the French news agency AFP that obstacles to an agreement revolve around the extent of the halt in Iran's uranium enrichment programme.
The EU would like Iran to suspend all its uranium enrichment activities indefinitely, whereas Iran is pushing for a six-month freeze covering only certain stages of the process, the diplomat said.
The IAEA has threatened to report Iran to the UN Security Council over fears it may have a secret weapons programme.
Mr Solana said he believed there would be no need to follow that course if a deal was reached before the IAEA's next meeting on 25 November.
'Difficult but useful'
Iran denies that it is seeking to acquire a nuclear arsenal.
A spokesman for the IAEA told the BBC the two sides were expected to report back to each other on Wednesday.
German Foreign Ministry spokeswoman Antje Leendertse said the talks had been "difficult but useful".
White House spokesman Scott McClellan said the international community was determined to prevent Iran developing nuclear weapons "through diplomatic means".
Iranian Foreign Minister Kamal Kharrazi told state-run television the move was a "positive trend" which would help "build confidence".
But an influential conservative newspaper, the Islamic Republic, has rejected the idea of a deal with Europe, and defended Iran's right to arm itself with nuclear weapons.
Regional analyst Sadeq Saba says this is the first time a publication seen as close to the Iranian supreme leader, Ayatollah Ali Khamenei, has openly called for a nuclear Iran.