A US official said Tuesday that Iran has agreed to drop demands to block some UN nuclear inspections, the latest concession that could herald a revival of a 2015 nuclear deal.

The account comes after US officials also said Iran was relaxing its insistence that Washington remove the powerful Revolutionary Guards from a terrorism blacklist, a key dispute in indirect negotiations that had stalled after a year and a half.

Iran has sought to shut down a probe by the International Atomic Energy Agency (IAEA) into three undeclared nuclear sites where the watchdog says it has questions related to previous nuclear work.

In June, Iran disconnected a number of IAEA cameras after the UN body's board censured Tehran for not adequately explaining previous traces of uranium.

A senior official in President Joe Biden's administration said Iran has "made concessions on critical issues" and that reports of new US compromises were "categorically false."

"In addition to the nuclear constraints Iran would have to implement, the IAEA would again be able to implement the most comprehensive inspections regime ever negotiated, allowing it to detect any Iranian effort to pursue a nuclear weapon covertly," the official said on condition of anonymity.

"Much of that international monitoring would remain in place for an unlimited amount of time."

Both sides are expected to cast the other as bending further in the effort to resume the 2015 deal, which is deeply unpopular both with some Iranian hardliners and the US Republican Party.

Former US Republican president Donald Trump unilaterally withdrew from it in 2018.

Biden took office vowing to restore the agreement, but faced insistence from Iran on lifting all sanctions imposed by Trump -- including new measures such as blacklisting the Revolutionary Guards, the clerical regime's elite ideological unit.

An official from the European Union, which has brokered the indirect talks, said Iran has agreed to defer discussion on the Revolutionary Guards until after restoration on the package.

But the United States will reportedly ensure it will not penalize outside actors solely for dealings with the Revolutionary Guards, which has wide interests across the Iranian economy.

The return of the United States to the accord would mean an end to Trump's unilateral sanctions on Iran selling its oil, at a time of high concern over energy prices.