Iran on Tuesday presented world powers with what it dubbed a potential breakthrough to end a decade-long standoff over its nuclear drive, and held landmark talks with US officials.
All sides underscored the positive atmosphere of the revived negotiations under the new administration of President Hassan Rouhani -- who succeeded conservative Mahmoud Ahmadinejad in August -- though Western negotiators said they were still examining the details of what Iran had put on the table.
Rouhani, seen as more moderate, has pledged transparency on the nuclear programme and engagement with the international community to try to get the punishing international sanctions against Iran lifted.
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 easing sanctions.
The global players and Iran's archfoe Israel fear that Tehran's atomic programme is a disguised effort to build a nuclear bomb, a claim it denies.
Underscoring the change in tone, senior Iranian negotiator Abbas Araqchi and his US counterpart Wendy Sherman met Tuesday evening, after a day of P5+1 negotiations in Geneva.
That marked the first direct nuclear talks between the Islamic republic and Washington since 2009.
"The discussion was useful, and we look forward to continuing our discussions in tomorrow's meetings with the full P5+1 and Iran," a senior US State Department official said after the hour-long meeting.
"It demonstrates our continued commitment to bilateral engagement," the official added, underlining that Rouhani and US President Barack Obama had spoken by telephone during last month's UN General Assembly in New York.
Iranian Foreign Minister Mohammad Javad Zarif and the six powers also met then, accompanied by a landmark two-way meeting with US Secretary of State John Kerry.
EU foreign policy chief Catherine Ashton met one-on-one with Zarif later in the evening to take stock of the first day of negotiations, officials said.
Earlier Tuesday, Zarif, Araqchi and their team had made an hour-long presentation to the P5+1 -- in English, for the first time, which Western diplomats said underlined Tehran's new tone.
"The proposal that we have introduced has the capacity to make a breakthrough," Araqchi said afterwards, telling reporters it was "very comprehensive" but that all parties had agreed to keep it under wraps.
He nonetheless indicated what was not on the table, with Iranian state news agency IRNA quoting him as saying that an additional protocol to the nuclear Non-Proliferation Treaty allowing unannounced inspections was not part of the offer.
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Iran has drawn other red lines, saying it will not accept any demand to suspend uranium enrichment or ship its 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".
'Ball remains in Iran's court'
Pressed to reveal what Iran had pledged to do, Western officials 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.
US State Department spokeswoman Jen Psaki said that "since the technical conversations and discussions are ongoing, I don't think we'd characterise it as a breakthrough at this stage".
"However, it certainly is positive that there was enough information to have technical discussions," she added.
In Geneva, EU spokesman Michael Mann said discussions had been "very detailed", highlighting the "very different" atmosphere from previous talks.
Iran's Araqchi also praised the "very positive environment" and said the "reaction was good" to the closely guarded proposal.
"We are very serious. We are not here symbolically, to waste our time. We are serious for target-oriented negotiations," Araqchi said.
Israel -- believed to be the Middle East's only nuclear armed state -- warned Tuesday against accepting "cosmetic concessions" that would not stop Iran from pursuing nuclear weapons.
It has not ruled out a military strike, and has warned the world not to fall for Rouhani's "sweet talk".
Western negotiators insist they are cautiously hopeful but not naive about Iran.
"The ball remains in their court," said Mann.
A senior US official said any easing of sanctions would be "targeted, proportional to what Iran puts on the table", with "concrete, verifiable actions" needed.