"My own assessment, talking to all of our colleagues, is that returning to mutual compliance, it remains possible," Blinken told reporters.

"We've seen, I would say, some modest progress in the last couple of weeks in the talks" in Vienna, he added.

Speaking at a joint news conference, German Foreign Minister Annalena Baerbock said that "urgent progress" was needed in talks aimed at reviving the 2015 nuclear deal.

"The window for finding a solution is closing," she said.

"The negotiations are in a decisive phase. We need urgent, urgent progress, otherwise we will not be successful in reaching a joint accord."

Blinken agreed it was a "decisive moment" in the negotiations, adding "time is running out" to reach a deal.

Their comments came a day after US President Joe Biden said it was "not time to give up" on the talks with Iran, insisting "there is some progress being made".

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

The talks on the accord known as the JCPOA resumed in November.

Blinken said last week that there were only "a few weeks left" to save the deal, and the US was ready to look at "other options" if the talks fail.

The deal offered Iran much-needed relief from sanctions that have crippled its economy, in return for curbs on its nuclear programme.

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

Tehran is seeking verification of the sanctions easing, as well as guarantees that Washington will not withdraw from the deal again.

Baerbock said progress was being hobbled by the fact that "Iran, parallel to the talks, unfortunately continues to turn the spiral of nuclear escalation".

Western powers have complained about slow process in the talks at a time when Iran has accelerated its nuclear work, for example increasing uranium enrichment.

The West wants Iran to meet a number of requirements including destroying its advanced centrifuges.