Iran's negotiators are expected to offer restrictions on its nuclear programme in return for at least a partial lifting of sanctions, at a new round of talks starting in Geneva on Tuesday, diplomats said.
The complexity of the proposals means a completed deal is unlikely at the end of two days of negotiations, as there will remain significant gaps between the Iranian and western negotiating positions, but diplomats pointed to a new level of engagement not seen for several years.
The Iranian foreign minister and lead negotiator, Mohammad Javad Zarif, posted a message on his Facebook account saying the Geneva talks were "the start of a difficult and relatively time-consuming way forward".
He said: "I am hopeful that by Wednesday we can reach agreement on a road map to find a path towards resolution. But even with the goodwill of the other side, to reach agreement on details and start implementation will likely require another meeting at ministerial level."
The Iranian road map for the talks was described in vague terms on Friday by Zarif's deputy, Abbas Araqchi, who said Tehran would negotiate "the form, amount, and various levels of [uranium] enrichment" but would not agree to shipping enriched uranium out of the country.