Talks to restore the 2015 accord between Tehran and world powers -- United States, France, Britain, Russia, China, and Germany -- began last year but stopped in June as Iran elected ultraconservative President Ebrahim Raisi.

The 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.

According to the top US diplomat, 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 2015 deal offered Iran much-needed relief from sanctions that have crippled its economy, in return for curbs on its nuclear program.

But then-US president Donald Trump's unilateral withdrawal from the deal in 2018 prompted Tehran to walk back on its commitments.

Trump's successor Joe Biden has backed a return, with Washington indirectly taking part in the European-brokered negotiations on reviving the so-called Joint Comprehensive Plan of Action (JCPOA).

After months of stalled talks, hosted in Vienna, Washington recently reported modest but still insufficient progress.

Reviving the accord "would be the best result for America's security," said Blinken. "But if we can't, we are looking at other steps, other options" with allies including in Europe and the Middle East.

Blinken has previously raised implicit threats of military action.

The other options have been "the subject of intense work as well in the past weeks and months," he said. "We're prepared for either course."