France on Monday warned Russia not to resort to blackmail over efforts to revive a nuclear deal between Iran and world powers, after Moscow demanded a U.S. guarantee that sanctions it faces over Ukraine would not hurt its trade with Tehran.

Russia raised the potential stumbling block on Saturday, just as months of indirect talks between Tehran and Washington in Vienna over reviving the pact struck in 2015 appeared headed for agreement.

In Tehran, a foreign ministry spokesperson said Iran was awaiting an explanation of the Russian demand via "diplomatic channels", adding however that the talks should not be affected by sanctions imposed on Moscow, whose contribution to negotiations so far had been constructive. 

On Saturday, a senior Iranian official speaking to Reuters had called Russia's move unconstructive.

Foreign Minister Sergei Lavrov said Russia wanted a written U.S. guarantee that Moscow's trade, investment and military-technical cooperation with Iran would not be hindered by Western sanctions imposed since Russia invaded its neighbour.

A French presidency official told reporters late on Sunday that diplomats tended to treat each issue on its merits and not conflating them.

"Because otherwise, in reality, it's just blackmail and not diplomacy," he told reporters.

Western officials say compartmentalising the Iran nuclear dossier has been possible due to a common interest in avoiding a non-proliferation crisis.

They are trying to ascertain if 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 2015 deal between Iran and the United States, France, Britain, Germany, Russia and China, eased sanctions on Tehran in return for limiting Iran's enrichment of uranium, making it harder for Tehran to develop material for nuclear weapons.

It fell apart after President Donald Trump withdrew the United States in 2018.

'TRYING IT ON'

The French presidency official urged Russia to assess what was at stake in Vienna, "that is to say Iran's return to respecting its obligations under the JCPOA," referring to the 2015 deal by its formal name, the Joint Comprehensive Plan of Action.

A European diplomat added: "The Russians are really trying it on and the Iranians aren’t happy although of course not saying too much publicly. We’re trying to find a way through.",

Iran's top security official Ali Shamkhani said on Monday that negotiators were evaluating new components that had affected the Vienna talks and that Iran was adapting initiatives to accelerate an agreement.

U.S. Secretary of State Antony Blinken sought on Sunday to dispel talk of obstacles, saying the sanctions imposed on Russia over Ukraine had nothing to do with the nuclear deal.

Diplomats said Washington and Tehran were also still trying to resolve other outstanding issues, which were also stalling an agreement.

European negotiators have temporarily left the talks as they believe they have gone as far as they can and it is now up to the two main protagonists to agree, three diplomats said.

Russia's concerns about the impact of Western sanctions on its dealings with Iran follow a push by senior Iranian officials for deeper ties with Russia since the election of Iran's hardline president Ebrahim Raisi last year.

Iran's top authority, Supreme Leader Ayatollah Khamenei, has been calling for closer ties with Russia due to his deep mistrust of the United States.

Reuters