Lavrov told a press conference that Russia was ready to accept a draft document restoring the agreement. But he said there were “problems that have arisen recently from the point of view of Russia’s interests”.

Under the new deal, the Biden administration is expected to lift sanctions on Iran imposed by President Donald Trump after he withdrew the United States from the deal in 2018, and Iran will have to return to restrictions on its nuclear program. The original agreement, known as the Joint Comprehensive Plan of Action, would then be reinstated.

President Biden has pledged to return to the JCPOA, and diplomats have spent months in Vienna negotiating the details.

Russia’s invasion of Ukraine added a new layer of complication by shifting the parties’ geopolitical calculations. Russia was a signatory to the original agreement along with the United States, Iran, China, Britain, France and Germany.

Given what Lavrov called the “avalanche of aggressive sanctions” imposed by the West on Russia, the country may find itself unable to benefit from the opening of trade and investment opportunities with the ‘Iran.

He said Russia wanted “written guarantees at the minimum level from the Secretary of State” that the new sanctions will not affect Russia’s right to “free and full trade and economic and investment cooperation and military-technical cooperation with Iran”.

It was unclear whether such guarantees would be possible, but the demand dampened hopes of an imminent deal.

Russia had previously sought and obtained assurances that Ukraine-related sanctions would not apply to Russia’s role in overseeing the implementation of the JCPOA, which gives Russia the responsibility to withdraw and store Iran’s surplus enriched uranium stockpiles, diplomats say.

Lavrov’s comments on Saturday suggest Russia is asking for a much broader exemption, a senior Western diplomat said. If so, it would be “a serious problem for the negotiation”, he said.

With almost all the final details now ironed out, any new demands would inevitably prolong the negotiation and “in the current environment, any delay is risky,” the diplomat added.