Negotiators from Iran and six world powers met Thursday to broker a deal that could see Tehran freeze its disputed nuclear programme in exchange for some relief from sanctions.
Officials have said that a long-awaited agreement on curbing Iran's nuclear ambitions may be finally within reach, after years of fruitless talks were given fresh momentum by the election of Iran's new President Hassan Rouhani, seen as a relative moderate.
In their second meeting in Geneva in less than a month, negotiators from the United States and five other global powers sat down with Iranian officials for two days of talks aimed at hammering out the "first step" of an agreement.
Iran's lead negotiator in Geneva, Deputy Foreign Minister Abbas Araqchi, suggested that a major hurdle had already been crossed.
"The other side accepted Iran's proposed framework, the first step, the last step and the steps in between, and now we have to discuss the details of these steps," Araqchi said in comments shown on Iranian state television.
But in a sign of potential troubles ahead, Israel slammed the possible deal, with Prime Minister Benjamin Netanyahu saying it would be "a mistake of historic proportions".
Negotiators from the so-called P5+1 group -- Britain, China, France, Russia, the United States and Germany -- met with Iranian officials for a first session in the late morning.
The sides then broke off and Iran held bilateral talks with the Europeans, Russia, China and the United States.
A European diplomat said the talks had been "long, substantive and constructive".
A senior State Department official said the US delegation had met with the Iranians for about an hour and had "a substantive and serious conversation".
But a spokesman for EU diplomatic chief Catherine Ashton, who is chairing the talks for the P5+1, said it was too early to speak of an end game.
"I can't give you any sort of final verdict yet," said the spokesman, Michael Mann.
"We hope that they have come with the same attitude (as previous talks), that they want to make progress.... The ball is in their court."
In a possible indication the talks were making headway, Iran's Foreign Minister Mohammad Javad Zarif, who opened the negotiations with Ashton, cancelled a planned trip to Rome.
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'Complicated, difficult and intensive' talks
Araqchi said Zarif decided to stay in Geneva because talks "have entered a complicated, difficult and intensive" phase.
Both sides have said recent talks have been the most productive in years but admit that reaching a deal will not be easy.
Zarif nonetheless sounded an optimistic note earlier this week, saying he believed it would be "possible" to reach an agreement.
The meeting is the second since Rouhani took office in August pledging to resolve the nuclear dispute and lift sanctions by engaging with world powers.
Iran is anxious for relief from crippling economic sanctions that have cut oil revenues by more than half, caused the value of the rial to plunge and pushed inflation above 40 percent.
The West is also keen to seize a rare opportunity to build bridges with Iran after decades of hostility, opening the door to engaging with Tehran on other issues like the conflict in Syria, where Iran has backed President Bashar al-Assad against insurgents.
Last month's talks in Geneva -- held in English for the first time -- saw Iran reportedly outline a two-stage process that would resolve the dispute within a year.
Speaking to journalists in Geneva on the eve of this week's talks, a senior US official said Washington is willing to offer Iran limited sanctions relief if it agrees to take a "first step" to stop advancing its nuclear programme.
"What we're looking for now is a first phase, a first step, an initial understanding that stops Iran's nuclear programme from moving forward... and that potentially rolls some of it back," said the official.
"We are prepared to offer limited, targeted and reversible sanctions relief," the official said.
US officials have indicated that relief could include unfreezing some Iranian assets outside the country, but that the "core architecture" of sanctions will remain in place.
The P5+1 group has held years of talks with Tehran on its uranium enrichment, which Western powers suspect may be aimed at developing nuclear weapons.
Iran has repeatedly denied this, insisting its nuclear programme is only for generating electricity and for medical purposes.
The six powers have been pushing Iran to freeze its enrichment efforts, reduce stockpiles and lower its capacity to produce nuclear material.