Chief negotiators from the United States and Iran were due to hold indirect talks in Qatar on Tuesday, bidding to overcome obstacles that have stalled attempts to revive a nuclear deal.

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

The talks in the Qatari capital are the start of a process to "unblock" the Vienna talks, European Union foreign affairs spokesman Peter Stano said, adding they would start on Tuesday.

"We managed to unblock the process and we are going to move forward, and as a first step at this stage we have these proximity talks," he said in Brussels.

"That means indirect talks between Iran and the United States on finding the way how to move forward."

The deal aimed at curbing Iran's nuclear ambitions has been hanging by a thread since 2018, when then US president Donald Trump unilaterally withdrew from it and began reimposing harsh economic sanctions on America's arch-enemy.

US President Joe Biden's administration has sought to return to the agreement, saying it would be the best path ahead with the Islamic republic, although it has voiced growing pessimism in recent weeks.

The talks in Doha will take place indirectly, with the delegations in separate rooms and communicating via intermediaries. The US and Iran do not have diplomatic relations.

Earlier Malley met Qatar's Deputy Prime Minister and Foreign Minister Mohammed bin Abdulrahman Al Thani to discuss "joint diplomatic efforts to address issues with Iran", the US embassy in Doha tweeted.

Diplomatic hub 

Bagheri and his delegation have arrived in Doha, Iran's IRNA state news agency reported.

Sheikh Mohammed also discussed the Iran talks with his French counterpart Catherine Colonna in a phone call on Tuesday, the official Qatar News Agency said.

Qatar's foreign ministry said in a statement that it hopes the "indirect talks will be culminated in positive results that contribute to the revival of the nuclear deal signed in 2015".

EU foreign policy chief Josep Borrell said during a visit to Tehran on Saturday that the Iran-US talks would be held in a Gulf country to avoid confusion with the broader talks in Vienna.

Qatar, which has better relations with Iran than most Gulf Arab monarchies, has sought a role as a diplomatic hub.

It also hosted talks between the United States and the Taliban before the withdrawal of American troops from Afghanistan in August last year.

The Vienna talks began in April last year but hit a snag in March following differences between Tehran and Washington, notably over Iran's demand that its Islamic Revolutionary Guard Corps be removed from a US terror list.

"I always say that the time is not really on our side, so we should really move forward very quickly," Stano said, referring to the Qatar meetings.

"But again, it's not up to the European Union or (Borrell) as a coordinator to determine the tempo because everything depends on the willingness of the participants to find the necessary way forward."