US says 'ball in Iran's court' on reviving nuclear deal

White House national security adviser Jake Sullivan said on Sunday the United States had begun to communicate with Iran over the country's detention of American citizens, calling the matter a "complete and utter outrage".

WASHINGTON (BLOOMBERG) - Iran and the United States sparred over how to revive a nuclear deal, reflecting the challenge ahead for the Biden administration even as nuclear inspectors persuaded Iran to temporarily allow some wider monitoring.

Teheran over the weekend renewed its demand that the new US administration rejoin the accord and lift crippling Trump-era sanctions on the Iranian economy before talks can resume.

By contrast, US National Security Adviser Jake Sullivan said "the script has been flipped" because President Joe Biden has offered to re-engage with the Islamic Republic.

"It is Iran that is isolated diplomatically now, not the United States, and the ball is in their court," Mr Sullivan said on CBS's "Face the Nation".

While each side is demanding the other take the first step towards returning the US to the accord abandoned by former President Donald Trump, officials said on Sunday (Feb 21) that the two governments have been in indirect contact since Mr Biden took office a month ago.

Mr Sullivan said Mr Biden's offer to talk with Iran about how it can return to full compliance with the nuclear accord "still stands," even after Teheran rebuffed the overture on Friday.

He said the sides are in contact on the release of US citizens detained by Iran, a dispute that will be critical to resolving broader mutual hostility.

Foreign Ministry spokesman Saeed Khatibzadeh confirmed to state television that Iran has received messages from the US via the Swiss embassy in Teheran.

Iran is poised on Tuesday to suspend a voluntary agreement that gives the International Atomic Energy Agency expanded inspections powers over its nuclear sites. Teheran tapped the brakes on the escalation on Sunday with a temporary understanding that partially offsets its revocation of the voluntary accord.

The compromise follows US and European pleas for Iran to continue adhering to the IAEA's so-called Additional Protocol to give diplomacy a chance.

"This is a temporary solution in the hope we can return to what we had," IAEA Director General Mariano Grossi said after talks in Teheran with Iranian Foreign Minister Javad Zarif.

The protocol that Iran is revoking allowed unannounced inspections of nuclear material and installations where undeclared activity is suspected.

While the IAEA will still be allowed into Iran to account for declared nuclear stockpiles, oversight of places such as machine shops and mines involved in uranium enrichment will be lost, as will camera surveillance of Iranian nuclear sites.

Earlier on Sunday, Mr Zarif reiterated that any talks with the US "will not be about changing the terms of the agreement, regional issues or missile issues." They also would have to address a guarantee that the US won't quit the deal a second time, Mr Zarif said on state-run Press TV.

The 2015 deal between Iran, the US, Russia, China, the UK, France and Germany locked in restrictions on the Iranian nuclear programme, notably uranium enrichment, in exchange for sanctions relief. Iran has always said its nuclear programme is entirely civilian.

Mr Trump unilaterally took the US out of the deal in 2018 and violated its terms by reimposing and expanding a severe sanctions regime on Iran's economy. Iran responded by ramping up its nuclear activities. The crisis almost triggered a war between the two countries.

Mr Biden is effectively continuing Mr Trump's "maximum pressure" policy by choosing to maintain sanctions and denying Iran access to at least US$10 billion (S$13 billion) trapped in overseas accounts because of the US penalties, Mr Zarif said.

While Iran will return to full compliance with the original deal "the minute" the US formally rejoins, it will further increase its stockpile of 20 per cent enriched uranium until the US returns, he said.