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Iran said on Saturday it had agreed to hand over documents on its nuclear programme to the UN’s atomic watchdog, in a step that could help clear the way for a broader deal between Tehran and world powers.
But Russia's involvement in the negotiations, at a time when it faces increasing isolation over the war in Ukraine threw up a potential stumbling block, as Foreign Minister Sergey Lavrov demanded guarantees from the US that sanctions on Moscow would not impede trade with Iran.
"It would have all been fine, but that avalanche of aggressive sanctions that have erupted from the West - and which I understand has not yet stopped - demand additional understanding by lawyers above all," Mr Lavrov said.
Iranian nuclear chief Mohammad Eslami said he had struck an agreement with the visiting head of the International Atomic Energy Agency, Rafael Grossi, to exchange documents with the UN body by mid-June.
The documents “relate to outstanding questions between Tehran and the agency”, said Mr Eslami, which notably include an IAEA investigation into uranium traces at several old but undeclared sites in Iran.
Mr Grossi told the same press conference a wider restoration of the Iran nuclear deal “may not be possible” unless Tehran’s bilateral issues with the IAEA are resolved.
The director general, who also held talks with Iran’s foreign minister on Saturday, described his understanding with Tehran as an agreement to "work together, to work very intensively" and take a “practical, pragmatic approach” to overcome their remaining differences.
Although these IAEA-Iran issues are separate from the talks in Vienna on restoring the so-called Joint Comprehensive Plan of Action, Iran has indicated it wants the first out of the way before it agrees to the second.
Mr Grossi said he will not drop the IAEA’s inquiries into undeclared nuclear sites to enable a JCPOA deal, but said on Saturday the two sets of discussions were “closely interrelated” and that “one cannot ignore the other”.
Mr Grossi said it was hard to imagine any agreement to revive the 2015 nuclear deal if the UN nuclear watchdog's efforts to resolve open issues in Iran by June fail.
"My impression is that it would be difficult to imagine you can have a cooperative relationship as if nothing had happened if the clarification of very important safeguards were to fail," Mr Grossi told a news conference.
Diplomats in Europe have described the JCPOA talks as entering an endgame after 11 months of negotiations between Iran, Britain, Germany, France, China, Russia and indirectly the United States.
However, Mr Lavrov said in his intervention on Saturday that "problems have appeared recently from the point of view of Russia's interests" following its invasion of Ukraine.
He demanded a written guarantee from top US officials that the wave of sanctions being imposed on Russia would "not in any way harm our right to free, fully-fledged trade and economic and investment cooperation and military-technical cooperation with Iran".
A US State Department spokesperson said that the Russian sanctions are not related to the 2015 Iran nuclear deal, and should not have any impact on a potential revival of that agreement.
"The new Russia-related sanctions are unrelated to the JCPOA and should not have any impact on its potential implementation," the spokesperson said. "We continue to engage with Russia on a return to full implementation of the JCPOA. Russia shares a common interest in ensuring Iran never acquires a nuclear weapon. "
Britain’s top negotiator Stephanie Al-Qaq had said on Friday European officials were briefly returning home from Vienna to update their political bosses, but said: “We are close”.
But western diplomats have signalled the talks cannot drag on forever because of the pace of Iran’s continuing nuclear activities.
Under the 2015 deal, Iran agreed to limits on its nuclear activities in exchange for the US and other powers lifting sanctions. The agreement was meant to stop Tehran from developing a nuclear weapon, something it denies wanting to do.
But Iran has openly flouted these limits, for example by enriching and stockpiling uranium far above the thresholds set out in the deal, since former US president Donald Trump withdrew from the pact in 2018 and re-imposed sanctions.
The IAEA said this week Iran’s enriched uranium stockpile had reached 15 times the limit set out under the JCPOA.