Iran has questioned whether a July deadline for a nuclear deal with world powers will be met after talks with senior US officials in Geneva.
Iran's talks with six major powers on curbing its nuclear programme in exchange for an end to Western sanctions could be extended for six months if no deal is reached by a July 20 deadline agreed by all parties, a senior Iranian official said.
Abbas Araqchi, Iran's deputy foreign minister, said the talks were "fruitful" but that "divergences remain and the consultations are going to continue".
He said the discussions were "intense and difficult, but they are taking place in a positive atmosphere," the AFP news agency reported.
Araqchi said it was "too soon to judge" whether more time was needed.
"But the good thing is all parties are seriously committed to meet that goal," he said of the July 20 target.
"Whether we can do it or not is something else," he told Iranian media in the Swiss city.
Fuelling further doubts on meeting the July deadline, Laurent Fabius, French foreign minister, told France Inter
radio: "We are still hitting a wall on one absolutely fundamental point, which is the number of centrifuges which allow enrichment.
"We say that there can be a few hundred centrifuges, but the Iranians want thousands, so we're not in the same
Degree of refinement
Iran, which says its nuclear programme is peaceful and aimed at generating electricity - has about 19,000
centrifuges, of which roughly 10,000 are operating, according to the UN nuclear agency.
Enriched uranium can have both civilian and military uses, depending on the degree of refinement.
Reporters in Washington asked Jen Psaki, State Department spokeswoman, about Fabius' comments.
She said the focus should be on the actual negotiations taking place behind closed doors, not on what parties to the talks are saying publicly.
"I've seen those remarks," she said. "We feel our efforts should be directed towards the negotiations happening behind the scenes on the tough issues and not on public demands."
France has long held out for strict terms in the negotiations. Based on Psaki's remarks, it appeared that Fabius was not necessarily speaking for the other five powers - the US, Germany, Britain, China and Russia.
While an extension is possible under the terms of the talks, experts believe both Iran and the international powers may face domestic political pressures to argue for better terms during this extra time period, further complicating negotiations, the Reuters news agency reported.
The six powers and Iran will meet again in Vienna for another round of negotiations June 16-20.