US Secretary of State John Kerry has raised doubts over whether Iran is prepared to conclude a final deal with Western powers on dismantling its disputed nuclear programme, but has urged US lawmakers not to impose new sanctions on the country.
"I came away from our preliminary negotiations with serious questions about whether or not they're ready and willing to make some of the choices that have to be made," Kerry told the US House of Representatives foreign affairs committee on Tuesday.
"Has Iran changed its nuclear calculus? I honestly don't think we can say for sure yet. And we certainly don't take words at face value," Kerry said.
The top US diplomat, who helped hammer out an interim six-month deal with the country to freeze parts of its nuclear programme, said "believe me this is not about trust".
"Given the history we are all rightly sceptical about whether people are ready to make the hard choices to live up to this."
But he stressed Iran's seriousness would be put to the test over the six months set out in the interim deal hammered out last month in Geneva.
Iran has denied accusations it is seeking to acquire a nuclear weapon under the guise of its civilian atomic energy programme.
Kerry said "we now have the best chance we've ever had to test this proposition without losing anything" and he urged lawmakers to hold off imposing new sanctions on Tehran to give negotiators time to work.
"I'm not saying never [...] If this doesn't work we're coming back and asking you for more. I'm just saying not right now."
Two US senators - Democrat Robert Menendez and Republican Mark Kirk - are finalising a new Iran sanctions measure that they hope to introduce before Congress goes on its year-end recess.
Republican Senator John McCain, who said he hoped senators could "get an agreement in the next day or two", dismissed the idea that introducing new sanctions legislation now would hurt the interim agreement.
"It's supposed to be a six-month deal," he said of the legislation, which would aim to punish Iran if it reneged on its part of the deal that it reached last month with members of the so-called P5+1 group of Western powers.
Fellow Republican Senator Lindsey Graham said the new sanctions would not take effect until after the six months, and would "basically tie to the UN resolutions".
Kerry said the world faced a crossroads, "a hinge point in history": one path could lead to a resolution of concerns about Iran's nuclear programme, the other could lead to conflict.
He warned that if the US went ahead with new sanctions, it risked angering Washington's P5+1 partners and could also give Iran an excuse to flout the deal.