Iran has insisted that a revived nuclear accord, which diplomats say is close, is conditional on the Islamic Revolutionary Guard Corps (IRGC) being taken off a US terrorist list.

But Malley, the US envoy for Iran talks, told a conference in Doha: "The IRGC will remain sanctioned under US law and our perception of the IRGC will remain."

The Guards are on the list because of Iran's action supporting the Syrian government, Yemen's Huthi rebels and the Lebanese group Hezbollah.

EU foreign policy chief Josep Borrell told the Doha Forum on Saturday that a revived nuclear accord would be sealed in "a matter of days".

Iran has been engaged in negotiations to revive the accord with Britain, China, France, Germany and Russia directly and the United States indirectly since April 2021.

The European Union's coordinator for the Vienna talks, Enrique Mora, is in Tehran where he is hoping to find a way to hammer out remaining differences.

But Malley cautioned at the Doha Forum of political and business leaders that a deal was not "inevitable" and not "just around the corner".

"We're pretty close," he said of the negotiations, but added: "We've been pretty close now for some time. And I think that tells you all you need to know about the difficulty of the issues."

Just before Malley spoke, Sayyid Kamal Kharrazi, a former Iranian foreign minister and now advisor to the country's supreme leader, stressed the importance of the Guards -- Iran's ideological army.

"Yes it's imminent but it depends on the political will of the United States," Kharrazi said of the deal intended to replace one that the United States withdrew from in 2018.

"IRGC, certainly it has to be removed," he said.

"The IRGC is the national army and the national army cannot be listed as a terrorist group," he said.

The United States designated the Guards as a "foreign terrorist organisation" under then-president Donald Trump in April 2019.

It came the year after Trump unilaterally withdrew the United States from the 2015 nuclear deal, known formally as the Joint Comprehensive Plan of Action.

Kharrazi also said Iran wanted guarantees that the United States would not withdraw again from the deal. Malley said there could be no guarantee after President Joe Biden's term ends.