"The ball remains in their court, but not for long," Blinken told reporters in Pittsburgh where he took part in US-EU trade talks.

"There is a limited runway on that, and the runway is getting shorter."

Blinken reiterated that President Joe Biden was willing to return the United States to a 2015 accord in which Iran drastically scaled back nuclear work in return for promises of economic relief.

Former president Donald Trump trashed the deal known formally as the JCPOA and reimposed sweeping sanctions, which Iran wants removed before it undoes a series of steps out of compliance that it took to protest the pressure campaign.

"Simply getting back to the terms of the JCPOA at some point will not be sufficient to recapture the benefits of the agreement because of the progress Iran has made," Blinken said.

Blinken said that the Biden administration has been engaged "in very good faith for many months" in indirect talks in Vienna with Iran on returning to compliance.

Iran requested a break in talks in June due to a political transition as the ultraconservative Ebrahim Raisi became president, replacing Hassan Rouhani who entered the nuclear deal and favored better relations with the West.

No date has been set for talks to resume, although Raisi has said that he backs diplomacy to end sanctions.

On a visit to France, which remains in the nuclear deal, Iranian foreign ministry spokesman Saeed Khatibzadeh said that analytical work by Tehran will end soon.

"I assume that in a few weeks we are in a position to set a date with our friends in Europe," he told a gathering in Caen.

"We are not going to waste even a minute to get back to Vienna," he said.

Khatibzadeh had earlier told Le Monde newspaper that the new Iranian administration would return to the talks more quickly than Biden's did.

Negotiations had resumed on April 6 in Vienna, 77 days after Biden took office on January 20.

However, "it has only been 50 days since the new Iranian government took office," he said.