Iran and the UN nuclear watchdog said on Saturday they had agreed an approach for resolving issues crucial to reviving the country’s 2015 nuclear accord with world powers – but new Russian demands may delay a deal.

The announcement came shortly before Russia said it would seek guarantees from the US before it backs the deal, potentially scuppering hopes an agreement could be wrapped up quickly.

International Atomic Energy Agency (IAEA) chief Rafael Grossi said the UN agency and Iran “did have a number of important matters that we needed . . . to resolve”, but that they had now “decided to try a practical, pragmatic approach” to overcome them.

Atomic Energy Organisation of Iran president Mohammed Eslami said the two sides had come to the “conclusion that some documents which need to be exchanged between the IAEA and the Iranian organisation should be exchanged” by May 22.

However, addressing reporters on his return to Vienna, Grossi said he didn’t “have a crystal ball” to predict how the process would end.

The aim is to settle outstanding questions that the IAEA has about the past presence of nuclear material at undeclared sites in Iran.

Grossi stressed that there was “no artificial deadline, no pre-defined outcome”.

“If Iran does not cooperate . . . I will not cease to put questions and continue the process,” he added.

Tehran has said the closure of the matter is a pre-requisite for a deal to revive the 2015 nuclear accord with world powers, also known as the Joint Comprehensive Plan of Action (JCPOA).

Grossi’s visit to Tehran came a day after Britain, one of the parties to parallel talks on the deal in Vienna, indicated an agreement was close.

But Russia’s foreign minister Sergei Lavrov said on Saturday that Moscow, itself slapped with sanctions over its military offensive in Ukraine, would seek guarantees from Washington before backing the nuclear deal.

Lavrov said Russia had requested that the US give it written guarantees that Ukraine-related sanctions “will not in any way harm our rights to free, fully-fledged trade and economic and investment cooperation, military-technical cooperation with Iran”.

Russia is party to the ongoing talks in the Austrian capital to restore the agreement along with Britain, China, France and Germany. The US is participating indirectly.

As with the original JCPOA in 2015, Moscow is expected to play a role in the implementation of any fresh deal with Iran, for example by receiving shipments of enriched uranium from Iran.

The 2015 nuclear deal has been hanging by a thread since then US president Donald Trump pulled out in 2018 and reimposed sweeping sanctions, including on Iran’s vital oil and gas exports.

The landmark accord was aimed at guaranteeing Tehran could not develop a nuclear weapon – something it has always denied wanting to do.

Iran said last week that it was ready to raise its crude exports to pre-sanctions levels within one to two months of a deal being signed.

The coming days are seen as pivotal by the West because of the rate at which Iran is making nuclear advances.

Its stockpile of enriched uranium has now reached more than 15 times the limit set out in the 2015 accord, the IAEA said last week.