"I do not think the negotiations will reach a conclusion tonight," Abbas Araqchi, a deputy foreign minister, was quoted as saying by the Mehr news agency after an afternoon session with the P5+1 group of world powers.
- Netanyahu: I pledge Iran will not get nuclear weapon
- Israel, Gulf in 'strange alliance' against Iran
- Iran negotiator: No deal without right to enrich uranium
The remarks came at a gathering in Geneva with political directors from Britain, China, France, Russia and the United States plus Germany.
Araqchi said that the talks had been "serious" and had focused on remaining "points of difference."
He was referring to Iran's insistence that the six recognize its right to enrich uranium.
Araqchi refused to speculate whether a deal would be clinched on Friday, saying: "One cannot judge about tomorrow."
The talks – the third since President Hassan Rohani, a reputed moderate, took office in August – seek a long-elusive deal to allay Western suspicions that Iran's civil nuclear program conceals a drive for a weapons capability.
Earlier, Foreign Minister Mohammad Javad Zarif had expressed hope for an interim agreement but called for flexibility and balanced positions by the P5+1 group.
Zarif said the talks were being held in "a positive atmosphere" and had gone into "details." He did not elaborate.
- Receive Ynetnews updates
directly to your desktop