US Secretary of State Antony Blinken on Monday said Iran could cut its nuclear weapon breakout time to "a matter of weeks" should it continue to escalate its breaches of the 2015 nuclear deal.
“The programme is galloping forward,” Mr Blinken told the House of Representatives' foreign affairs committee.
“The agreement pushed [breakout time] into a year or more. It’s now down, by published reports, to a few months at best.
"And if this continues, it will get down to a matter of weeks, exactly what we sought to avoid and what the agreement stopped.”
Since former president Donald Trump withdrew from the nuclear deal in 2018, Iran has steadily increased its breaches of the accord.
It installed advanced centrifuges at its Natanz nuclear site while stockpiling 2.5 kilograms of 60 per cent enriched uranium, 90kg of 20 per cent enriched uranium and 5,000kg of 5 per cent enriched uranium.
President Joe Biden has offered to lift the Trump administration's sanctions on Iran as part of a “compliance for compliance” return to the deal.
But Washington and Tehran are slowly arguing over which sanctions the US will need to lift and what activities Iran will have to suspend through indirect, months-long talks in Vienna.
Mr Blinken again questioned Iran's willingness to return to compliance with the deal.
“It remains unclear whether Iran is willing and prepared to do what it needs to do to come back into compliance, so we’re still testing that proposition,” Mr Blinken said.
The Iranian elections scheduled for next week could further delay nuclear diplomacy as hardline cleric Ebrahim Raisi, a staunch critic of the deal, is overwhelmingly favoured to win after the Guardian Council barred more moderate candidates from running.
But supreme leader Ayatollah Ali Khamenei has the final say on all security matters and has allowed the Vienna talks to proceed in hopes of seeing the sanctions removed.
Mr Blinken also criticised Iran’s support for Hamas after the militants sent a barrages of rocket attacks into Israel last month following weeks of mounting tension in Jerusalem.
But he said that the rockets Hamas fired did not come from Iran.
“The best public assessment is that in its most recent incident, most of the rockets were indigenously produced in Gaza by Hamas,” Mr Blinken said.
“Iran’s support for Hamas has been a persistent problem for a long time, a problem that existed before the nuclear deal, that continued during the nuclear deal and continues today despite the so-called maximum pressure campaign when we’re out of the deal."