A landmark agreement to contain Iran's nuclear programme is moving beyond reach despite hours of talks between John Kerry, the US secretary of state, and his Iranian counterpart.
Although the two countries do not have formal diplomatic ties, Mr Kerry held marathon negotiating sessions with Mohammad Javad Zarif, the Iranian foreign minister, in Geneva as the two men strove to conclude a "first step agreement".
This would have frozen Iran's nuclear programme in return for easing sanctions, with the aim of winning time for a permanent settlement of a confrontation which threatens a new war in the Middle East.
Mark Dubowitz, an Iran sanctions expert at the Foundation for Defense of Democracies think-tank who has advised Congress and the Obama administration calculated that the sanctions relief would total some $20 billion – almost doubling the estimate of available foreign exchange reserves to Iran.
"If this deal went through you would be handing $20 billion dollars to the Iranian regime that you would never get back and which could be used to further the very nuclear programme we are trying to stop.
"In effect, we would be giving $20 billion dollars-worth of irreversible sanctions relief for a collection of entirely reversible concessions on Iran's nuclear programme."