Iran and the International Atomic Energy Agency said Saturday after talks in Tehran that they agreed on an approach to resolve issues crucial in efforts to revive the country's 2015 nuclear deal.

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

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 Organization 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.

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

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 this 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 next few days are widely seen as a decisive point for negotiations on reviving the accord formally known as the Joint Comprehensive Plan of Action, or JCPOA.

"We are close," British delegation head Stephanie Al-Qaq said Friday.

Russia seeks US guarantees 

But Russia's Foreign Minister Sergei Lavrov said on Saturday that Moscow, itself slapped with sanctions over its invasion of 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 United States is participating indirectly.

Iranian international relations analyst Fayaz Zahed said the government needed to be very careful that Moscow did not scupper a deal in defence of its own interests.

"Now that Russia is under sanctions, it is perhaps no longer interested in resolving the Iran nuclear issue, a position that could be very damaging," he said.

Grossi vowed this week that the IAEA would "never abandon" its attempts to get Iran to clarify the presence in the past of nuclear material at several undeclared sites.

Iran has said the closure of the probe is necessary to clinch a deal.

Behrouz Kamalvandi, deputy head of the Iranian atomic agency, told state television he was hopeful Iran would reach an agreement with the IAEA during Grossi's visit.

Grossi is expected to hold a news conference on his return to Vienna.

Work ongoing

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 this week.

Several observers believe the West could leave the negotiating table and chalk the deal up to a failure if a compromise is not reached this weekend.

The EU has been chairing nuclear deal negotiations and the bloc's foreign policy chief Josep Borrell said on Friday he "hopes to have results this weekend" to "resurrect the agreement".

He stressed there was "still work ongoing".

On Thursday, US State Department deputy spokeswoman Jalina Porter said negotiators were "close to a possible deal", but that "a number of difficult issues" remained unresolved.

However, "if Iran shows seriousness, we can and should reach an understanding of mutual return to full implementation of the JCPOA within days", she said.