Global powers were due to gather with Iran today, seeking to slot into place the last pieces of a complex puzzle to curtail Tehran's nuclear programme as diplomats said tentative agreement on some parts had been reached.
US Secretary of State John Kerry will lead the team of six world powers - who hope an end to more than a decade of nuclear tensions with Iran may be in sight - for their first full plenary session of the latest round of talks.
While diplomats said some key points appeared to have been resolved to ensure Iran cannot make a covert dash for a nuclear bomb, they cautioned that the outlines of a political understanding were not yet fully agreed.
"We are here because we believe a deal can be done," British Foreign Secretary Philip Hammond told reporters in Lausanne late yesterday as he became the last of the P5+1 group of foreign ministers to arrive in the Swiss town for the talks.
"But it has to be a deal which puts the bomb beyond Iran's reach," he said, adding he hoped for success before tomorrow's midnight deadline.
"There can't be any compromise about that," Hammond insisted, before meeting with his Chinese, French, German, Russian and US counterparts, as well as EU foreign policy chief Federica Mogherini.
As negotiators in Switzerland raced against the clock, Israeli Prime Minister Benjamin Netanyahu launched a blistering attack on the "dangerous" deal.
One Western diplomat said Iran had "more or less" agreed to slash the number of its sophisticated centrifuges from 20,000 to 6,000 and to ship abroad most of its stockpile of nuclear material.
But Iran's chief negotiator Abbas Araqchi insisted: "There is no question of sending the stocks abroad."
Iranians have expressed guarded optimism that after 18 months of tortuous negotiations and two missed deadlines, a breakthrough may be at hand.
"Getting to an accord is doable. Solutions have been found for numerous questions. We are still working on two or three issues... The talks are in their final phase and are very difficult," Araqchi said.
The aim is to agree broad outlines for an accord by midnight tomorrow and then flesh out a series of complex annexes containing all the technical details by June 30.
The mooted deal would see Iran scale down its nuclear programme and allow unprecedented inspections of its remaining activities.
Its underground facility at Fordo would also likely be barred from uranium enrichment, diplomats said.
