Iran's chief nuclear negotiator Ali Bagheri Kani arrives at Palais Coburg where closed-door nuclear talks with Iran take place in Vienna, Austria, February 28, 2022.
Iran's chief nuclear negotiator Ali Bagheri Kani arrives at Palais Coburg where closed-door nuclear talks with Iran take place in Vienna, Austria, February 28, 2022. Reuters / LEONHARD FOEGER

Indirect talks between Iran and the United States on reviving the 2015 Iran nuclear deal are close to reaching an agreement, the chief British envoy said on Friday as she and her French and German colleagues flew home to brief ministers.

"We are close. E3 negotiators leaving Vienna briefly to update Ministers on state of play. Ready to return soon," Stephanie Al-Qaq said on Twitter, referring to the chief British, French and German diplomats involved in the talks.

The Europeans' departure raised the possibility that the United States and Iran, which have talking indirectly because Tehran refuses to meet face to face, might be preparing to sit down together though diplomats said there were no such plans.

"As far as I know, the Iranians are not ready for direct talks," Russian envoy Mikhail Ulyanov told reporters. "We will have a deal maybe in the middle of next week. We are talking about the last efforts before crossing the finish line."

Negotiators have worked for 11 months to try to revive the 2015 deal under which Iran limited its nuclear program to make it harder to obtain fissile material for a bomb - an ambition Tehran denies - in return for relief from economic sanctions.

Western powers and Russia have for almost a year worked closely to revive the accord but the war in Ukraine is creating a sense of urgency to conclude talks before cooperation becomes more difficult.

Then-U.S. President Donald Trump reneged on the pact in 2018, restoring U.S. sanctions that have slashed Iran's oil exports, and prompting Iran to begin violating the deal's nuclear limitations about a year later.

On Thursday, Ulyanov had said he did not think the talks would now collapse and a ministerial meeting - typically where a deal would be blessed - was likely but he could not say if it would be on Saturday, Sunday or Monday.

Myriad pieces must fall into place for a deal to happen.

One wildcard is an effort by the International Atomic Energy Agency (IAEA) to resolve questions about nuclear material that the Vienna-based agency suspects Iran failed to declare.

The IAEA has found particles of processed uranium at three apparently old sites that Iran never declared and has repeatedly said Tehran has not provided satisfactory answers.

IAEA Director-General Rafael Grossi will travel to Tehran on Saturday hoping to agree on a process that would lead to the end of the investigation, potentially clearing a way for the wider agreement, diplomats said.

Another open question is the fate of Western prisoners in Iran.

While U.S. officials say talks on the two issues are separate, the top U.S. negotiator has said Washington unlikely to strike a nuclear deal unless Tehran frees four U.S. citizens that the United States says Iran holds hostage.

Iranian Foreign Minister Hossein Amirabdollahian hinted a nuclear agreement may be close but said the West's "haste" to reach one "cannot prevent the observance of Iran's red lines."

"We are ready to finalise a good and immediate agreement," Iranian media quoted Amirabdollahian as telling the EU's top diplomat Joseph Borrell by telephone. "Most of Iran's requests have been considered in the upcoming agreement."