The International Atomic Energy Agency has presented a draft deal to Iran and three world powers to reduce Tehran's stockpile of enriched uranium.
Iran declined to say if it would endorse the plan, which Western diplomats said would require it to send 1.2 tonnes of its known 1.5-tonne reserve of low-enriched uranium to Russia and France by the end of the year.
The material would be converted into fuel for a nuclear medicine facility in Tehran.
The Islamic Republic's envoy to the International Atomic Energy Agency hinted that his government may seek amendments.
Western diplomats suggested this could jeopardise the deal if they overstepped 'red lines' set to create confidence that Iran is not pursuing a nuclear weapons option.
However, an outright rejection seemed less likely as this could revive Western pressure for harsher UN sanctions on Iran.
'We have to thoroughly study this text and ... come back and reflect our opinion and suggestions or comments in order to have an amicable solution at the end of the day,' Iranian Ambassador Ali Asghar Soltanieh told reporters.
'We welcome this event; we are fully cooperating.'
Three days of talks in Vienna failed to finalise the deal as the IAEA and the three powers - France, Russia and the US - had wanted.
Western diplomats said this was because Iran raised many questions about fundamental aspects of the plan, which it had already agreed to in principle.
IAEA Director General Mohamed ElBaradei announced the draft deal after the Vienna talks.
'I have circulated a draft agreement that in my judgment reflects a balanced approach to how to move forward. The deadline for the parties to give, I hope, an affirmative action is Friday,' he told reporters.
Meanwhile, a representative of Israel's Atomic Energy Commission has held several meetings with an Iranian official to discuss nuclear issues in the region.
'There were several meetings between a representative of our commission and an Iranian official in a regional context,' Yael Doron of the commission told AFP.
'These meetings were held behind closed doors', she said, adding that they were organised by Australia.
She declined to give further details of the talks, the first between the two arch foes to be officially disclosed since the Shah of Iran was deposed in 1979.
