United States Secretary of State Antony Blinken says there are only “a few weeks left” to save the 2015 Iran nuclear deal before Tehran’s advancements will become too difficult to reverse.

Blinken spoke on Thursday as negotiations in Vienna between Tehran and the other signatories of the 2015 deal, from which former US President Donald Trump unilaterally withdrew in 2018, continued.

The US has been participating in the talks indirectly, with Washington and Tehran, despite trading charged rhetoric, recently reporting modest gains after months of near-total deadlock. The newest round of talks resumed in November.

“We have, I think, a few weeks left to see if we can get back to mutual compliance,” Blinken said in an interview with US public radio station NPR.

“We’re very, very short on time,” because “Iran is getting closer and closer to the point where they could produce on very, very short order enough fissile material for a nuclear weapon,” he said.

Blinken added that Tehran has made nuclear advances that “will become increasingly hard to reverse because they’re learning things, they’re doing new things as a result of having broken out of their constraints under the agreement”.

The nuclear deal offered direly needed international sanctions relief to Iran in exchange for curbs on its nuclear programme.

Trump reimposed a “maximum pressure” sanctions campaign after withdrawing from the agreement, and Tehran has since increasingly flouted the restrictions in the deal, arguing it is no longer beholden to the agreement following the US withdrawal.

US President Joe Biden has made returning to the deal a top priority, while newly elected Iranian President Ibrahim Raisi, despite holding more hardline positions than his predecessor, is eager to find relief from crushing sanctions.

In an interview with Al Jazeera in early January, Iranian Foreign Minister Hossein Amir-Abdollahian said a return to the deal could be reached if “all forms of sanctions stipulated in the nuclear agreement” were lifted – an apparent softening of the government’s previous calls for a complete lifting of all sanctions, even those imposed on human rights grounds.

On Thursday, Blinken said reviving the accord “would be the best result for America’s security”.

“But if we can’t, we are looking at other steps, other options” with allies including in Europe and the Middle East, he added.

Those “other options” – often seen as an implicit threat of military actions – have been “the subject of intense work as well in the past weeks and months”, Blinken said.

“We’re prepared for either course.”