Iran has said the closure of the probe is necessary in order to clinch a deal to revive the 2015 deal with world powers on its nuclear programme.

However, on Wednesday International Atomic Energy Agency (IAEA) Director General Rafael Grossi insisted that it would "never abandon a process... because of a political reason".

"This is not how the IAEA works," he told reporters at a press conference.

The IAEA has been pressing Tehran for several years for explanations regarding indications that nuclear material was previously present at four different locations in Iran.

While much of the activity concerned is thought to date back to the early 2000s, sources say that one of the sites, in the Turquzabad district of Tehran, may have been used for storing uranium as late as the end of 2018.

Iranian Foreign Ministry spokesman Saeed Khatibzadeh said on Monday that unless this issue is closed, "we can't think of the possibility of an agreement about the return of the US" to the Iran nuclear deal.

Diplomats from Britain, China, France, Germany, Iran and Russia restarted talks in late November to revive the 2015 accord, also known as the JCPOA.

The US has been taking part indirectly.

The accord began disintegrating when former US President Donald Trump withdrew from it 2018 and re-imposed sanctions on Iran.

This week is widely being seen as critical for the success or failure of the talks, with German Chancellor Olaf Scholz saying Wednesday that a decision "cannot be postponed any longer".

When asked by reporters what it would take for him to close the issue of the sites, Grossi replied simply: "Iran to cooperate with me" in the form of a "full process to clarify" outstanding questions.

He didn't rule out a possible visit to Tehran soon in order to discuss the issue.

Russia's ambassador to the UN in Vienna Mikhail Ulyanov said the negotiations were "one minute from the finish line" and that the war in Ukraine had not affected Russia's co-operation with other participants.

"They need us very much," Ulyanov said, adding: "Pragmatically they don't want to spoil relations" on an issue of common interest.