With all sides praising the changed tone from new Iran's team, senior negotiators backed by experts on nuclear affairs and sanctions were to comb through the details of what Iran billed as a breakthrough offer.
Iranian negotiators and counterparts from the European Union-chaired P5+1 group -- the United States, Britain, France, China and Russia, plus Germany -- were set to meet behind closed doors from 9:30 am (0730 GMT) on Wednesday.
The Geneva talks, which began Tuesday, ended a six-month freeze sparked by Iran's refusal to curb uranium enrichment in exchange for easing the punishing unilateral sanctions.
On Tuesday, Iranian Foreign Minister Mohammad Javad Zarif and his team made an hour-long presentation to the P5+1 -- in English, for the first time, which Western diplomats said underlined the new mood.
"The proposal that we have introduced has the capacity to make a breakthrough," Iran's lead negotiator Abbas Araqchi said, telling reporters it was "very comprehensive" but that all parties had agreed to keep it under wraps.
He indicated what was not on the table, however, with Iranian state news agency IRNA quoting him as saying that an additional protocol to the nuclear Non-Proliferation Treaty allowing snap inspections was not part of the offer.
Iran's other red lines include suspending uranium enrichment or shipping stockpiles of purified material abroad.
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".
Araqchi said the "reaction was good" to the proposal.
"We are very serious. We are not here symbolically, to waste our time," he added.
Western officials at home and in Geneva were tight-lipped.
"We're not going to negotiate this in public or go into the details of what was in their proposal," said White House spokesman Jay Carney.
State Department spokeswoman Jen Psaki said that "since the technical conversations and discussions are ongoing, I don't think we'd characterize it as a breakthrough at this stage".
"However, it certainly is positive that there was enough information to have technical discussions," she added.
A senior US official in Geneva said any easing of sanctions would be "proportional" to what Iran pledged, with "concrete, verifiable actions" needed.
Also in Geneva, EU foreign policy spokesman Michael Mann said discussions had been "very detailed". He praised the "very different" atmosphere from previous talks but saying the ball remained in Iran's court.
Last month, Iran’s President Hassan Rouhani and US President Barack Obama held telephone talks during the UN General Assembly in New York, where Zarif held a landmark two-way meeting with US Secretary of State John Kerry on the sidelines of a P5+1 gathering.
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