"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.