In a blow to European mediators, Iran requested a new pause in the talks in Vienna, which aim to bring the United States back into the 2015 agreement and roll back nuclear activities. The Islamic republic stepped up its nuclear projects after the US withdrawal.

The talks had just resumed in late November after a five-month break following the election of a new hardline government in Iran.

"There has been some technical progress in the last 24 hours, but this only takes us back nearer to where the talks stood in June," Britain, France and Germany, known as E3, said in a statement.

"We are rapidly reaching the end of the road for this negotiation."

Underlying Western concerns are fears that Iran will soon have made enough progress that the 2015 accord -- under which it was promised economic relief in return for drastic curbs on its nuclear work -- will be obsolete.

Enrique Mora, the EU official chairing the talks, called for a "sense of urgency" and for talks to resume before the end of the year.

"We are not talking anymore about months, we are talking about weeks," Mora said.

- Limited progress -

Former US president Donald Trump pulled out of the deal in 2018 and imposed sweeping sanctions including a unilateral US ban on Iran's oil sales, vowing to bring the US adversary to its knees.

President Joe Biden supports a return to the agreement negotiated by predecessor Barack Obama, formally known as the Joint Comprehensive Plan of Action, but has been frustrated by the pace of resurrection efforts.

"It's not going well in the sense that we do not yet have a pathway back into the JCPOA," Biden's national security advisor, Jake Sullivan, said of the talks.

"We are paying the wages of the disastrous decision to leave the deal back in 2018," he said.

But Sullivan, speaking at the Council on Foreign Relations in Washington, said recent days "have brought some progress at the bargaining table".

Another US official said the latest round was "better than it might have been" and "worse than it should have been".

The official called for a "very significant acceleration" and said the United States was ready to return before New Year's.

"If it takes this much time to agree on a common agenda, imagine how much time it will take to resolve the issues on that agenda," he said.

Russia, which along with China is also in the talks, said negotiators agreed to start from where they left off in June before Iran requested a break for its elections.

The latest round was "successful in a sense that it prepared sound basis for more intensive negotiations," envoy Mikhail Ulyanov wrote on Twitter.

Tehran's chief negotiator Ali Bagheri said there were "hard and intense negotiations" to agree on the "bases" for further talks which will take place "in the near future".

- 'Plan B'? -

The Biden administration has said it is willing to lift sanctions but only if Iran returns to compliance.

Amid the deadlock, the United States has increasingly spoken of a "Plan B" of pressure if talks fail.

A group of former officials including Obama's defense secretary Leon Panetta and retired general David Petraeus in a joint statement urged Biden to arrange high-profile military exercises or other actions to strike fear into Iran.

"Without convincing Iran it will suffer severe consequences if it stays on its current path, there is little reason to hope for the success of diplomacy," they wrote, while also backing humanitarian assistance.

Earlier this year, Tehran also began restricting some inspection activities by the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).

Tehran and the Vienna-based IAEA announced Wednesday that they had reached agreement on replacing the cameras at the TESA nuclear complex in Karaj, west of Tehran, after they were damaged in a June attack Iran blames on Israel.

IAEA director general Rafael Grossi on Friday voiced concern that a camera memory unit remained missing from the complex.

"This is why we are asking them, 'Where is it?' I'm hopeful that they are going to come up with an answer because it is very strange that it disappears," Grossi said.

"We have ways to try to reconcile the facts on the ground with what Iran is going to be telling us," he added.

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