A landmark agreement to contain Iran's nuclear programme hung in the balance last night, despite hours of talks between US Secretary of State John Kerry and his Iranian counterpart.
Although the two countries do not have formal diplomatic ties, Mr Kerry held marathon negotiating sessions in Geneva with Mohammad Javad Zarif, the Iranian foreign minister, as the two men strove to conclude a "first-step agreement".
But Mr Kerry was under pressure from Israel – which pre-emptively condemned the possible deal – and from some of his other allies.
Laurent Fabius, the French foreign minister, rejected a draft agreement, and issued a public warning against a "sucker's deal".
At issue was precisely what nuclear work Iran would halt under an interim deal, and which Western sanctions would be relaxed.
An outline agreement is believed to have specified that Iran would stop enriching uranium to 20 per cent purity – close to weapons grade – and convert its stockpile of the material into harmless oxide.
Iran would also scale down its enrichment of uranium to the 3.5 per cent level needed for nuclear power stations.
The big obstacle appears to have been the future of Iran's plutonium reactor at Arak, which could provide another route to nuclear weapons capability. This facility, currently being built, is due to become operational next year.
Iran would have promised not to activate Arak for a period of perhaps six months. But the Western powers, particularly France, wanted Tehran to go further and freeze all work on the plant. Given the reactor would not have been ready for another year anyway, some regarded a delay in switching it on as meaningless.
But experts questioned whether Arak should have assumed such importance. "Arak is not supposed to come on line until after the middle of next year, so it's not so urgent that it couldn't be dealt with in the next six months," said Mark Fitzpatrick, a non-proliferation specialist at the International Institute for Strategic Studies.
"The French may be concerned that Iran may try to speed up the start-up date . . . I'm not sure if this is the time for it to be a deal-breaker."
As for the sanctions that America was prepared to ease, about €2.2bn of frozen Iranian assets would have been released. Restrictions on Iran's gold, petrochemical and car industries would also have been lifted.
Assuming that exports of those goods returned to their pre-sanctions level, this would have been worth another €12bn, meaning that the total package would have netted Iran about $19.5bn (€14.6bn) – four per cent of its economy
President of Iran Hassan Rouhani said: "The West should not miss this unique opportunity. Our nation is participating in the Geneva negotiations with strong will and determination."
Hopes of an agreement had risen when Mr Kerry broke off from a tour of the Middle East to join the meeting in Geneva on Friday. He was followed by Mr Hague, Mr Fabius and German Foreign Minister Guido Westerwelle.
Negotiations with Iran are handled by the "P5 plus 1", consisting of the UN Security Council's five permanent members – America, Britain, France, Russia and China – along with Germany. Normally, the talks are delegated to officials. By Friday night, however, four of the six members were represented by their foreign ministers.
Anxious not to be left out, Sergei Lavrov, the Russian foreign minister, flew to Geneva yesterday, followed by Wang Yi, his Chinese counterpart. By the afternoon, the full cast of foreign ministers from the "P5 plus 1" had gathered for the first time.
Mr Kerry began his vital meeting with Mr Zarif and Baroness Ashton, the EU's foreign affairs representative, who chairs the "P5 plus 1".
The US and Iran have not had diplomatic relations since the Islamic Revolution of 1979. But Mr Kerry and Mr Zarif talked from 7pm until midnight, spending longer in the same room than any holders of their respective jobs for 34 years. Before them was a draft of what would have been the first formal agreement between their countries since 1979.