The visit by the head of the International Atomic Energy Agency comes with the Vienna-based organisation seeking to get Iran to increase cooperation over its nuclear activities.

Grossi was greeted at the airport by Behrouz Kamalvandi, the spokesman for the Atomic Energy Organization of Iran and is due to meet its head, Mohammad Eslami, as well as other Iranian officials during his two-day visit.

A confidential IAEA report seen Tuesday by AFP said uranium particles enriched up to 83.7 percent -- just under the 90 percent needed to produce an atomic bomb -- had been detected at Iran's underground Fordo plant about 100 kilometres (60 miles) south of the capital.

Tehran denies wanting to acquire atomic weapons, and last week said it had not made any attempt to enrich uranium beyond 60 percent purity, noting that "unintended fluctuations... may have occurred" during the enrichment process.

The discovery came after Iran had substantially modified an interconnection between two centrifuge clusters enriching uranium, without declaring it to the IAEA.

The IAEA tweeted on Thursday that Grossi would "travel to Tehran for high-level meetings at the invitation of Iran's government".

A diplomatic source told AFP that Grossi would also meet President Ebrahim Raisi to "relaunch the dialogue" on Iran's atomic work and to "reset the relationship at the highest level".

During the visit, the IAEA director general will try to find out more and obtain "strengthened access to the (Fordo) site and an increase in the number of inspections", the sources added.

On Thursday, France, a signatory to a 2015 deal that promised Iran relief from biting economic sanctions in exchange for curbs on its nuclear activities, described the new enrichment development "unprecedented and extremely serious".

The IAEA chief's visit comes amid deadlock in negotiations on reviving the landmark accord, known formally as the Joint Comprehensive Plan of Action or JCPOA.

- 'Greater cooperation' -

These restrictions, including the 3.67 percent enrichment threshold set out in the deal, were intended to prevent Iran from developing a nuclear weapon.

However, the United States unilaterally withdrew from the deal in 2018 under then president Donald Trump and reimposed sanctions, prompting Iran to suspend the implementation of its own commitments under the accord.

Negotiations aimed at reviving the deal started in 2021 but have been stalled since last year due to a number of diplomatic as well as geopolitical reasons.

Grossi's visit is being seen in Iran as another indication that a dialogue-based approach to resolving the nuclear standoff is possible.

"It is hoped that this trip will form the basis for greater cooperation and a clearer horizon between Iran and the IAEA," said Kamalvandi before Grossi's arrival.

Foreign Minister Hossein Amir-Abdollahian said in an interview with CNN that "the window for an agreement on negotiations to lift sanctions is still open, but this window will not be open for ever".

Depending on the outcome of Grossi's trip, the United States and the E3 -- European powers Britain, France and Germany -- will decide whether submit a draft resolution censuring Iran to the IAEA board of governors, which is due to convene next week in Vienna.

In November 2022, Iran was criticised for its lack of cooperation regarding traces of enriched uranium found at three undeclared sites.

Grossi's last visit to Iran took place in early March 2022, and was focused on the three undeclared sites.

The IAEA said Grossi would hold a news conference upon his return to Vienna late Saturday afternoon.