Iran's team said it received a good reception to its new plan to make headway in the dispute with global players, who fear Tehran's atomic programme is a disguised effort to build a nuclear bomb, a claim it denies.
"The session focused on technical aspects of Iran's proposal, many questions were put forward," an Iranian source close to the talks told AFP after the first day of discussions wrapped up in Geneva.
Iran's two-day meeting with the European Union-chaired P5+1 group -- the United States, Britain, France, China and Russia, plus Germany -- ends a six-month freeze sparked by its refusal to curb uranium enrichment in exchange for the easing of punishing international sanctions.
"The proposal that we have introduced has the capacity to make a breakthrough," senior Iranian negotiator Abbas Araqchi said, telling reporters it was "very comprehensive" but that all parties had agreed to keep it under wraps.
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EU foreign policy chief Catherine Ashton was to meet one-on-one with Iran's Foreign Minister Mohammad Javad Zarif later in the evening to take stock of the first day of negotiations, officials said.
Rouhani, seen as more moderate, has pledged transparency on the nuclear programme and engagement with the international community to try to get the sanctions lifted.
In what Western officials said was a sign of the new mood, the Iranian team today delivered a presentation in English for the first time.
Earlier, Zarif said Tehran's plan contained three steps that could settle the long-running nuclear standoff "within a year", with the first achievable "within a month or two, or even less".
A senior US State Department official added: "For the first time, we had very detailed technical discussions, which carried on this afternoon and will continue tomorrow."
Iran's Araqchi also praised the "very positive environment" and said the "reaction was good" to Iran's hour-long PowerPoint presentation.