US Secretary of State Antony Blinken urged Iran on Saturday to release an American citizen he said has been held for years as a "political pawn," as the nations inch toward restoration of a nuclear deal.

Emad Sharghi was sentenced to 10 years in prison on spying charges, Iranian media reported in January 2021, saying he was detained trying to flee the country.

Blinken said the Iranian-American venture capitalist has been held for four years, and that the "family has waited anxiously for the Iranian government to release Emad.

"Like too many other families, their loved one has been treated as a political pawn," the top US diplomat said in a post on Twitter. "We call on Iran to stop this inhumane practice and release Emad."

Robert Malley, the US special envoy for Iran, said on Saturday that Sharghi was arrested exactly four years ago.

"He was cleared of all charges, but then convicted in absentia, rearrested, and has now spent over 500 days in Evin Prison," Malley said. 

"Emad, the Namazis, and Morad Tahbaz must all be allowed to come home now."

More than a dozen citizens of Western countries are being held in Iran, even after Tehran allowed two British citizens to return home last month after years of detention and another to leave prison.

Those who remain behind bars, under house arrest or unable to leave Iran face an agonizing wait to see if a possible deal on the Iranian nuclear program will help their prospects.

In 2015, Washington and five other world powers inked a landmark agreement with Tehran to rein in the Islamic Republic's nuclear activities in exchange for sanctions relief.

Under the presidency of Donald Trump, the United States quit the deal in 2018 and reinstated economic sanctions against Tehran, which in response shrugged off restrictions imposed on its nuclear efforts. 

Months of negotiations in the Austrian capital Vienna aim to return Washington to the deal, including through the lifting of sanctions, and to ensure Tehran's full compliance with its commitments.

Negotiators say they are close to a conclusion, but have yet to finalize all points.