Iran's lead negotiator in talks with world powers said that Wednesday's first session would cover the "process of the negotiations" and "probably" a draft deal not until the next day.
"In the evening plenary, we will discuss the process of the negotiations and should there be progress, we will probably begin discussing the draft tomorrow," Abbas Araqchi told the Fars news agency.
Araqchi added "the lost confidence must be regained", in reference to the last round of talks 10 days ago in Geneva with the United States, China, Russia, France, Britain and Germany that failed to reach an accord.
Foreign Minister Mohammad Javad Zarif told the Mehr news agency meanwhile that his talks Wednesday afternoon with the P5+1 powers' chief negotiator and EU foreign policy head Catherine Ashton were "good".
"We had good talks and discussed how to continue the process (of negotiations). We have not yet entered discussing the draft," Zarif said.
The powers want the talks, the third since moderate Hassan Rouhani became president in August, to result in a "first phase" deal involving the freezing of certain of Iran's nuclear activities.
This would give time to hammer out a longer-term accord that would reduce the programme to a size acceptable to the international community while removing sanctions placed on the Islamic republic.
Iran says its nuclear programme is peaceful but many in the international community suspect Tehran wants nuclear weapons. Any deal is likely to be a hard sell among hardliners in the US and Iran, as well as in Israel.