By John Irish and Francois Murphy

VIENNA: Parties trying to revive the Iran nuclear deal scrambled on Wednesday to resolve last-minute Russian demands that threaten to scupper negotiations, diplomats said, with the United States appearing unwilling to engage with Russia on the matter.

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. Iran's top negotiator returned to Vienna on Wednesday 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 with several diplomats saying the nuclear deal was now broadly agreed.

But just as the final issues were being resolved, Russia presented a new obstacle by demanding written guarantees from the United States that Western sanctions targeting Moscow over its invasion of Ukraine would not affect its trade with Iran.

"There is still no resolution to the Russia demands," said one Western diplomat after Russian envoy Mikhail Ulyanov and the talks' coordinator, Enrique Mora of the European Union, held talks on Tuesday.

Mora and Ulyanov met again on Wednesday.

"Yet another meeting with EU Coordinator at the ViennaTalks Mr. Enrique Mora at the very last stage of diplomatic marathon towards restoration of JCPOA," Ulyanov said on Twitter, referring to the Joint Comprehensive Plan of Action.

U.S. Under Secretary of State for Political Affairs Victoria Nuland on Tuesday accused Russia of seeking to reap extra benefits from its participation in the effort to restore the nuclear agreement, but she said Washington would not be playing "Let's Make a Deal."

The Western diplomat said it was still not clear what the exact nature of Moscow's demands were, but they appeared broader than its nuclear commitments to revive the deal.

A European diplomat said Russia was demanding sweeping guarantees on trade between Moscow and Tehran.

They said the talks were now not likely to end this week.

Mora broke off informal meetings on Monday saying the time had come for political decisions to be taken to end the negotiations.

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 United States and Iran to agree on outstanding issues.

Iran's chief negotiator, Ali Bagheri Kani, returned to Tehran unexpectedly after Russian Foreign Minister Sergei Lavrov outlined Moscow's new demands. Iran's foreign minister said at the time that Tehran would not let its interests be harmed by "foreign elements".