International negotiators from seven world powers are continuing efforts to finalize an interim deal to curb Iran's nuclear program in exchange for sanctions relief.
A spokesman for the European Union's chief diplomat said Thursday's first round of talks with Iran's foreign minister were "very substantial."
The remarks came as international negotiators from Iran and the world powers meet for a second day in Geneva.
Catherine Ashton's spokesman said she and Javad Zarif were "getting down to detailed work" in the morning session and were to meet again later.
Ashton is representing a group of six world powers that includes the United States, Britain, China, France, Russia and Germany. They want an interim deal that calls for Iran to stop some of its enrichment activity and accept more inspections in return for limited sanctions relief.
Iranian Deputy Foreign Minister Abbas Araqchi said earlier Thursday that Iran will not suspend its uranium enrichment as part of any agreement. He also said serious negotiations cannot happen until trust is restored between the two sides.
"I think what we need is good will and real willingness and determination to resolve these differences, to bridge these differences. I believe a solution is at our reach, what we need is only good faith."
French Foreign Minister Laurent Fabius said in a television interview Thursday he hopes an agreement can be reached, but that it has to be firm and concrete.
"Our position has to do with the security in the region and world security. If there is nuclear proliferation, that is, if more and more countries have nuclear weapons, that is obviously dangerous. So we are -- and when I say 'we' I don't mean only France, it's all the countries, the international community -- so that there is a right to nuclear power but not to atomic weapons."
The latest talks began Wednesday, building on a first round of negotiations that ended two weeks ago. Analysts say those talks failed in large part because France said the preliminary deal under consideration did not sufficiently curb Iran's uranium enrichment program.
U.S. Secretary of State John Kerry vowed Wednesday that the United States would not accept any deal that lets Iran buy time to increase its nuclear capability.
The White House on Tuesday described the current talks as "an opportunity to halt the progress on the Iranian program...while testing whether a comprehensive resolution can be achieved."
That statement came after President Barack Obama asked key Senate leaders to hold off on any new sanctions against Tehran while the Geneva talks continue.