Russia and the US held talks on the sidelines of the Iran nuclear summit on Wednesday, with Moscow signalling that the two geopolitical rivals needed to work together to strike a deal in Vienna.
The face-to-face discussions came amid high tensions between Moscow and Washington linked to Russia's military movements on its border with Ukraine.
Mikhail Ulyanov, Russia's delegate to the talks, said his discussions with US envoy Robert Malley were an “important prerequisite for progress” in restoring the nuclear deal.
Mr Ulyanov said he hoped for “close consultations and co-ordinations” between the two delegations as world leaders try to revive the troubled pact.
Russia and the US were two of the original signatories to the Iran deal, along with Britain, France, Germany, China and Iran itself.
The US left the pact in 2018 and reimposed sanctions, prompting Iran to flout the limits on its nuclear activity that it agreed to under the deal.
The current talks, which are in their eighth round, are aimed at bringing both countries back into compliance. Mr Malley is not negotiating directly with Iran, with diplomats from the other countries shuttling between the two.
Washington on Tuesday expressed caution over upbeat comments by Iran and Russia, saying it was still too soon to say that Tehran had returned to the talks with a constructive approach.
Mr Malley's delegation will be in a better position to judge Iran's stance in the coming days, said State Department spokesman Ned Price.
Separately, US Secretary of State Antony Blinken spoke with his French, German and British counterparts on Wednesday, when they "discussed their shared concerns about the pace of developments in Iran’s nuclear programme as time runs short for Tehran to return to the [deal]," the State Department said.
Russia's call for unity comes amid continuing tensions over the Ukrainian border, with Moscow yet to call off its troop build-up despite repeated warnings of severe consequences if it invades.
It was announced this week that the two nuclear-armed powers would hold talks in Geneva on January 10 to discuss European security.
The Kremlin has proposed a new security treaty relating to Europe, which would prevent Nato from expanding closer to Russia's borders. The US has described some of Moscow's demands as unacceptable.