Negotiations to restore the 2015 agreement began earlier this year but stopped in June as Iran elected a new ultraconservative government. They resumed in late November with the latest round getting underway on Monday.

The aim is to bring back Washington, which left the deal in 2018, and curtail Tehran's nuclear activities, stepped up in response to the US withdrawal and reimposed sanctions.

"This negotiation is urgent... We are clear that we are nearing the point where Iran's escalation of its nuclear programme will have completely hollowed out the JCPoA," negotiators from Britain, France and Germany said in a statement, referring to the deal's official name by its acronym.

"That means we have weeks, not months, to conclude a deal before the JCPoA's core non-proliferation benefits are lost."

Besides the so-called E3 European countries, Iran, China and Russia are also taking part in the talks, while the United States is participating indirectly.

The 2015 deal offered Iran a lifting of economic sanctions in return for strict curbs on its nuclear programme aimed at ensuring it would not build an atomic bomb -- an ambition Iran has always rejected.

A year after the US withdrawal and reimposition of sanctions, Iran in turn began to gradually abandon its commitments, including by stepping up its enrichment of uranium though it continues to deny that it wants to acquire a nuclear arsenal.

- 'Unprecedented' enrichment -

On Saturday, Atomic Energy Organization of Iran director Mohammad Eslami said Tehran had no plans to enrich uranium beyond 60 percent, even if the Vienna talks fail.

Eslami said the enrichment levels were related to the needs of the country, in remarks published by the Russian news agency RIA Novosti.

In response, E3 negotiators said Tuesday that 60 percent enrichment was still "unprecedented for a state without nuclear weapons". Military-grade levels are around 90 percent.

"Its increasing 60 percent stockpile is bringing Iran significantly closer to having fissile material, which could be used for nuclear weapons," they said.

Iran's Foreign Minister Hossein Amir-Abdollahian was quoted by state news agency IRNA on Tuesday as saying the negotiations were "on a good track".

"With the goodwill and seriousness from the other parties, we can consider (reaching) a quick agreement in the near future," he said.

Moscow's ambassador to the UN in Vienna, Mikhail Ulyanov, said the working group on nuclear issues held a "useful meeting" on Tuesday, while lifting sanctions was also discussed informally.

"We observe indisputable progress," he wrote on Twitter.

EU diplomat Enrique Mora, who is chairing the talks, said on Monday that all sides were showing "a clear will to work toward the successful end" but that "very difficult" negotiations lay ahead.

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