Western powers on Tuesday warned Russia against wrecking an almost completed deal on bringing the United States and Iran back into compliance with the 2015 nuclear accord, as Iran's top negotiator was set to return from consultations in Tehran.

Eleven months of talks to restore the deal which lifted sanctions on Iran in return for curbs on its nuclear programme have reached their final stages.

But they have been complicated by a last-minute demand from Russia for guarantees from the United States that Western sanctions targeting Moscow over its invasion of Ukraine would not affect its business with Iran.

U.S. Under Secretary of State for Political Affairs Victoria Nuland said Russia was seeking to reap extra benefits from its participation in the effort to restore the nuclear agreement, but it will not succeed.

"Russia is trying to up the ante and broaden its demands with regard to the (nuclear deal) and we are not playing 'Lets make a deal'," Nuland, the No. 3 U.S. diplomat, told a Senate Foreign Relations Committee hearing.

Iran's top negotiator Ali Bagheri Kani is due back in Vienna on Wednesday after unexpectedly returning to Tehran on Monday for consultations, an Iranian and a European official said.

The talks' coordinator, Enrique Mora of the European Union, said on Monday the time had come for political decisions to be taken to end the negotiations.

"The window of opportunity is closing. We call on all sides to make the decisions necessary to close this deal now, and on Russia not to add extraneous conditions to its conclusion," Britain, France and Germany said in a joint statement to the U.N. nuclear watchdog's 35-nation Board of Governors.

Iran has sought to remove all sanctions and it wants guarantees from the United States that it will not abandon the agreement once more, after then-U.S. President Donald Trump walked out of the deal in 2018 and reimposed sanctions.

Diplomats have said until now that several differences still needed to be overcome in the talks, including the extent to which sanctions on Iran, notably its elite revolutionary guards, would be rolled back and what guarantees Washington would give if it were to again renege on the deal.

Two Western officials said there was now a final text on the table and those issues had been resolved.

While they couldn't rule out further last-minute surprises, they said the last big open question was whether Russia's demands were manageably narrow and limited to nuclear cooperation spelled out in the agreement, as Moscow's envoy to the talks has told other parties, or much broader, as Russian Foreign Minister Sergei Lavrov has described them.

"We are very close to an agreement. It is essential we conclude while we still can," France's Foreign Ministry spokeswoman Anne-Claire Legendre told reporters in a daily briefing.

"We are concerned by the risks that further delays could weigh on the possibility of concluding," she said.

COMMON INTEREST

Moscow threw the potential wrench in the works on Saturday, just as months of indirect talks between Tehran and Washington in Vienna appeared to be headed for an agreement, with Lavrov saying the Western sanctions over Ukraine had become a stumbling block for the nuclear deal.

The EU's Mora and Russia's top negotiator Mikhail Ulyanov held talks in Vienna on Tuesday evening, exchanging views on the "current developments and way ahead," Moscow's envoy said on Twitter.

Western officials say there is common interest in avoiding a nuclear nonproliferation crisis, and they are trying to ascertain whether what Russia is demanding regards only its commitments to the Iran deal. That would be manageable, but anything beyond that would be problematic, they say.

The new agreement would lead to Russia taking in excess highly enriched uranium that would be taken out of Iran to bring Tehran back into compliance with the original deal's caps on the purity and amount of the enriched uranium it is stockpiling.

Rosatom, a state-run company formed by Russian President Vladimir Putin in 2007, is key to that and has still not been added to Western sanctions.

U.S. Secretary of State Antony Blinken played down the issue during a visit to Estonia on Tuesday and said Russia and the United States still shared a desire to prevent Iran from acquiring a nuclear weapon.

European negotiators from France, Britain, and Germany had already temporarily left the talks as they believed they had gone as far as they could go and it was now up to the two main protagonists to agree on outstanding issues.