Mark Dubowitz, who has advised the Obama administration and is considered to be the architect of the sanctions regime devised by Congress, said that the Iranians had "systematically worn down" the US position.
While the US said 18 months ago that its goal was to dismantle the Iranian military nuclear programme, it is now looking for technical fixes to restrain Iran, Mr Dubowitz, the executive director of the Foundation for Defense of Democracies think-tank, said.
Those technical fixes are believed to include agreements on where uranium is stockpiled and the number of observer visits rather than a more substantive deal.
Mr Dubowitz said that Iran had been emboldened by the decision to ease the pressure of sanctions and the message from President Obama that there was no military solution to the issue.
"I'm a big believer that in negotiating with this regime you need significant coercive leverage," he said. "It was tough sanctions that got Iran to the table. Unfortunately the approach of confidence-building measures, giving concessions to change Iranian behaviour, has done exactly the opposite and completely backfired."
He added: "The net result is that Iran has hardened its positions, drawn red lines that it doesn't move from, which has . . . made it more difficult to get the Iranians to make that fundamental strategic decision that it is no longer interested in pursuing a nuclear weapons programme."
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