The United States and Iran are working on a two-phase deal that clamps down on Tehran's nuclear programme for at least a decade before providing it leeway over the remainder of the agreement to slowly ramp up activities that could be used to make weapons.
Officials from some of the six-power talks with Iran said details still needed to be agreed on, with US and Iranian negotiators meeting today for the third straight day ahead of an end-of-March deadline for a framework agreement.
US Secretary of State John Kerry joined the negotiations after arriving yesterday.
A breakthrough was not expected before Kerry returns to Washington later. Still, Western officials familiar with the talks cited long-awaited progress on some elements that would have to go into a comprehensive deal.
They described the discussions as a moving target, however, meaning changes in any one area would have repercussions for other parts of the negotiation.
The idea would be to reward Iran for good behaviour over the last years of any agreement, gradually lifting constraints on its uranium enrichment program and slowly easing economic sanctions.
Iran says it does not want nuclear arms and needs enrichment only for energy, medical and scientific purposes, but the US fears Tehran could re-engineer the programme to another potential use, producing the fissile core of a nuclear weapon.
The US initially sought restrictions lasting for up to 20 years; Iran had pushed for less than a decade. The prospective deal appears to be somewhere in the middle.
One variation being discussed would place at least 10-year regime of strict controls on Iran's uranium enrichment program. If Iran complies, the restrictions would be gradually lifted over the last five years of such an agreement.
Iran could be allowed to operate significantly more centrifuges than the US administration first demanded, though at lower capacity than they currently run.
Several officials spoke of 6,500 centrifuges as a potential point of compromise, with the US trying to restrict them to Iran's mainstay IR-1 model instead of more advanced machines.
It would also be forced to ship out most of the enriched uranium it produces or change it to a form that is difficult to reconvert for weapons use. It takes about 1 ton of low-enriched uranium to process into a nuclear weapon, and officials said that Tehran could be restricted to an enriched stockpile of no more than 300 kilogrammes (about 700 pounds).
