But Iranian officials said they were hoping for progress in the talks in Qatar, which come after international meetings to return to the deal hit a roadblock.

Iran agreed to curb its nuclear programme in return for sanctions relief in 2015, before then US president Donald Trump pulled out of the deal three years later.

"We hope that, God willing, we can reach a positive and acceptable agreement if the United States abandons the Trump method," Iranian government spokesman Ali Bahadori-Jahromi said.

He described the "Trump method" as "non-compliance with international law and past agreements and disregard for the legal rights of the Iranian people".

The indirect talks -- with the rival delegations sending each other messages from different parts of the same hotel -- come just two weeks before US President Joe Biden makes his first official visit to the region, with Iran high on his agenda.

Soaring oil prices and the lack of spare capacity could also make this an opportunity for Tehran to push for the lifting of sanctions on Iranian crude, said Alex Vatanka, director of the Iran programme at the Washington-based Middle East Institute think tank.

Foreign Minister Hossein Amir-Abdollahian said Iran was open to a deal in Doha, but wouldn't cross its "red lines".

"We are serious" in our desire to finalise an agreement, he said, stressing that his country would not retreat from the "red lines" it has drawn.

"If the American side has serious intentions and is realistic, an agreement is available at this stage and in this round of negotiations," he said, quoted by IRNA state news agency.

IRNA has previously described the "red lines" as lifting all sanctions as related to the nuclear agreement, creating a mechanism to verify they have been lifted, and making sure the US does not withdraw from the deal.

The indirect negotiations headed by US special envoy Robert Malley and Iran's Ali Bagheri come after more than a year of European Union-mediated talks in Vienna on a return to the 2015 agreement.

The deal gave Iran sanctions relief in exchange for curbs on its nuclear programme to guarantee that Tehran could not develop a nuclear weapon -- something it has always denied wanting to do.

It has been hanging by a thread since 2018, when Trump unilaterally withdrew from it and began reimposing harsh economic sanctions on America's arch-enemy.

burs/th/dv