World-power foreign ministers were due Sunday to make a final push to get Iran to agree to curtail its nuclear programme days before a looming deadline to agree the outlines of a deal.
On Saturday officials expressed guarded optimism that after 18 months of tortuous negotiations and two missed deadlines a breakthrough might be in sight for a deal ending 12 years of tensions.
"We`re moving forward," Iranian Foreign Minister Mohammad Javad Zarif, hunkered down with US Secretary of State John Kerry in Switzerland since Thursday, told reporters.
"I think we can in fact make the necessary progress to be able to resolve all the issues and start writing them down in a text," Zarif said.
German Foreign Minister Frank-Walter Steinmeier, who like French counterpart Laurent Fabius joined the negotiations on Saturday, said the talks were in the "endgame".
Steinmeier added however that "the final metres are the most difficult but also the decisive ones".
Russia`s chief negotiator, Sergei Ryabkov was quoted by the RIA Novosti news agency as saying the chances of a deal were "more than 50/50".
EU foreign policy chief Federica Mogherini, who arrived late Saturday, said negotiators "have never been so close to a deal" but added there remained "critical points" to resolve.Russian Foreign Minister Sergei Lavrov and China`s Wang Yi were reportedly due to join the talks on Sunday. Britain`s Philip Hammond was also expected, completing the line-up.
A senior US State Department official said that she expected the presence of all foreign ministers in Lausanne would allow "the pace to intensify".
The mooted deal, due to be finalised by June 30, would see Iran scale down its nuclear programme and allow unprecedented inspections of its remaining activities.
This, the powers hope, would extend the theoretical "breakout" time that Iran would need to produce the fissile core of a nuclear bomb to at least a year from several months at present.
This would occur through a combination of slashing the number of machines producing nuclear material, exporting its stocks of this material and limiting the development of newer, faster equipment.
But Iran, which denies wanting nuclear weapons, is insisting that in exchange for any dismantling of its activities, the powers lift sanctions that have choked its economy by strangling its oil exports and banks.The global powers insist on a gradual phasing out of sanctions over many years to ensure Iran sticks to its commitments. The issue of UN sanctions is proving particularly thorny, diplomats said.
"Everything is linked. If all the technical issues are resolved and the questions tied to the sanctions are not, then there is no deal," said Iran`s nuclear chief Ali Akbar Salehi.
"The brinksmanship in these negotiations will no doubt continue until the eleventh hour," said Ali Vaez, an expert at the International Crisis Group.
Kerry is under pressure to return from Lausanne with something concrete to head off a push by Republican lawmakers to introduce yet more sanctions, potentially torpedoing the whole negotiating process.
There are also wider issues at play.
Russia`s Ryabkov said US-supported airstrikes by Iran`s foe Saudi Arabia on Iran-backed rebels in Yemen was "having an impact on the atmosphere" in Lausanne.
"We hope that the situation in Yemen will not bring about a change in the position of certain participants," he said.