Senior envoys from Iran and six world powers are in Vienna to draft a broad agreement expected to place strict limitations on Tehran's nuclear program in return for ending sanctions against the Islamic Republic.
Iranian Foreign Minister Mohammad Javad Zarif said the process would be long and complicated.
"It will be a matter of agreeing on commas and full stops," he said, adding that three more meetings would be required until he and his counterparts from Britain, China, France, Russia, the United States and Germany could sign a deal in late July.
At this week's round, the five UN veto powers and Germany are being represented by their chief negotiator, EU foreign policy chief Catherine Ashton, as well as by senior diplomats from these six countries.
In this year's earlier rounds, both sides staked out their diverging positions on issues such as the extent of cutting back on Iran's uranium enrichment program and how to make sure that the country's Arak reactor does not produce plutonium.
The six powers are concerned that the uranium or plutonium could be used in weapons and allege that Iranian engineers have been developing key nuclear weapons components.
Tehran's leaders insist the nuclear program is geared only towards energy production and scientific research.
The drafting process will also have to deal with finding a sequence for lifting the sanctions.
Iran's envoys would like them to be scrapped as soon as possible, while some of the world powers would like to lift them step by step.
Iran has already accepted some nuclear cuts and some sanctions have been suspended under a preliminary deal reached in November last year.
