Iran and six world powers began expert-level talks on Monday to work out nitty-gritty details in implementing a landmark accord for Tehran to curb its disputed nuclear program in return for a limited easing of sanctions.
The preliminary accord is seen as a first step towards resolving a decade-old standoff over suspicions Iran might be covertly pursuing a nuclear weapons "breakout" capability, a perception that has raised the risk of a wider Middle East war.
Officials from Iran and the United States, France, Germany, Britain, China and Russia met at the Vienna headquarters of the U.N. nuclear agency, which will play a central role in verifying that Tehran carries out its part of the interim deal.
The outcome of the meeting is expected to determine when Iran stops its most sensitive nuclear activity and when it gets the respite in sanctions that it has been promised in return.
The International Atomic Energy Agency (IAEA) said it would have "some involvement" in the discussions, which are expected to continue on Tuesday. Media were barred from the floor where the meeting, held under tight secrecy, took place.
The talks are aimed at "devising mechanisms" for the Geneva accord's implementation, Iranian Deputy Foreign Minister Abbas Araqchi was quoted by state Press TV as saying. Iranian nuclear as well as central bank officials would take part, he said.
Western diplomats said detailed matters not addressed at the Nov. 20-24 talks in Geneva must be ironed out before the deal can be put into practice.
These include how and when the IAEA, which regularly visits Iranian nuclear sites to try to ensure there are no diversions of atomic material, will carry out its expanded role.
A start to sanctions relief would hinge on verification that Iran was fulfilling its side of the accord, they said.
The deal was designed to halt Iran's nuclear advances for a period of six months to buy time for negotiations on a final settlement of the standoff. Diplomats say implementation may start in January after the technical details have been settled.
Scope for easing the dispute peacefully opened after the June election of a comparative moderate, Hassan Rouhani, as Iranian president. He won in a landslide by pledging to ease Tehran's international isolation and win relief from sanctions that have severely damaged the oil producer's economy.
Diplomats caution that many difficult hurdles remain to overcome - including differences over the scope and capacity of Iran's nuclear project - for a long-term solution to be found.
In a sign of this, Israeli Prime Minister Benjamin Netanyahu pressed the powers on Sunday to take a hard line with Iran in negotiations on a final agreement, urging them to demand that Tehran abandon all uranium enrichment.
A day after President Barack Obama deemed it unrealistic to believe Iran could be compelled to dismantle its entire nuclear infrastructure, Netanyahu said Tehran should have to take apart all centrifuges used to refine uranium.
Israel sees Iran, which has repeatedly said it seeks only civilian energy from uranium enrichment, as a mortal threat. Iran says it is Israel, widely believed to be the Middle East's only nuclear-armed power, that threatens peace.
Under last month's pact, Iran will halt the activity most applicable to producing nuclear weapons - enrichment of uranium to a higher fissile concentration of 20 percent - and stop installing components at its Arak heavy-water research reactor which, once operating, could yield bomb-grade plutonium.
In the Vienna talks, government experts will also discuss details of which components Iran is not allowed to add to the Arak reactor under the deal, as well as issues pertaining to the sequencing of gestures by both sides, the diplomats said.
Officials from the office of European Union foreign policy chief Catherine Ashton, who coordinates talks with Iran on behalf of the six powers, were also at the meeting.
Additional reporting by Isabel Coles in Dubai; Editing by Mark Heinrich