US negotiators do not know whether Iran is willing to do a deal to scale back its nuclear programme, Secretary of State Antony Blinken said.
Mr Blinken said the US was committed to restoring the Iran nuclear deal, from which former president Donald Trump withdrew in 2018.
Talks between Iran and world powers have since taken place in Vienna aimed at bringing the parties back together.
Mr Blinken cautioned on Thursday that it was still unclear whether Iran was willing to strike a new deal.
"We've been engaged in Vienna for some weeks with our European partners, Russia and China, and indirectly with Iran," he told BBC's Radio 4 Today programme.
“We’ve demonstrated our seriousness of purpose in terms of wanting to get back into the so-called JCPOA [Joint Comprehensive Plan of Action]. Compliance for compliance. What we don’t yet know is whether Iran is prepared to make the same decision and move forward.”
He said Iran was nearing the point of being capable of producing nuclear weapons.
“Before the agreement was reached some years ago, Iran was on the path where in a matter of weeks it could produce fissile material for a nuclear weapon and leave us no time to do anything about it,” he said.
“Right now, unfortunately, Iran has itself lifted many of the constraints imposed on it by the agreement because we pulled out. It is now getting closer and closer again to that point where its breakout time is going to be down to a few months and eventually even less.”
Mr Blinken's warning came as imagery obtained by the International Institute of Strategic Studies released on Wednesday to The National revealed the construction of a new set of seven silos storing advanced new missiles in southern Iran.
Families of western citizens held in Iran urged officials to link negotiations with Tehran to the plight of their relatives.
At least four Britons, four Americans, two Germans, two French people, two Swedes and two Austrians are detained in Iran.
Among the British citizens is Nazanin Zaghari-Ratcliffe, whose family say a UK debt of £400 million ($550 million) owed to Iran is a major reason for her detention.
The UK denied a report on Iranian television over the weekend that suggested the British government had paid the debt, with Foreign Office minister James Cleverly indicating the UK was still discussing the issue with Iran.
Asked whether the US would oppose a UK decision to pay the debt, Mr Blinken said: “It’s a sovereign decision for the UK.”
The US was forced to deny a separate report that Washington had agreed to release $7 billion in frozen Iranian funds and to free four Iranians in exchange for the release of four Americans.
Germany, France and the UK are the European powers negotiating a possible return to the 2015 Iranian nuclear deal, which US President Joe Biden is keen to re-enter if Tehran meets certain conditions.
Under the original accord, the US lifted sanctions on Iran worth billions of dollars in exchange for Tehran winding up its nuclear programme.
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