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 accord. Iran's top negotiator returned to Vienna on Wednesday from consultations in Tehran.

Russia's envoy to the talks, Mikhail Ulyanov, dismissed any suggestion Moscow was holding up an agreement and said a final text had in any case not been completed.

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 there was broad agreement.

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.

Ulyanov said Moscow's demands had not received a positive reaction.

"In view of the new circumstances and wave of sanctions against Russia we have the right to protect our interests in the nuclear field and wider context," Ulyanov said.

He said the United States and the European Union had to make it clear that neither now or in the future sanctions could hit the implementation of nuclear projects in Iran as well as its trade and economic relations.

Ulyanov met the coordinator of the talks, Enrique Mora of the European Union, on Tuesday evening and again on Wednesday.

He said he would still have to report back to Moscow for a final decision after the text was finalised.

"There is no final text so how can our position delay anything if final negotiations are not finished," he said.

"A number of participants at this moment are not ready to confirm that the text is fully acceptable to them."

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