Rafael Grossi, who arrived in Tehran the previous evening, was to meet Iran's vice president Mohammad Eslami, who also heads the country's nuclear agency, official sources said.

Grossi's visit comes as talks remain locked in Vienna on saving Tehran's 2015 deal with major powers that promised it sanctions relief in return for curbs on its nuclear programme.

The landmark deal was torpedoed in 2018 by former US president Donald Trump's unilateral decision to withdraw the United States from it and impose a punishing sanctions regime.

Iran has since stepped away from many of its commitments, but the administration of US President Joe Biden has advocated a return to diplomacy to save the agreement.

A point of contention this year has been limits Iran has imposed on the IAEA's ability to monitor various of its nuclear facilities.

It has refused to provide real-time footage from cameras and other surveillance tools that the UN agency has installed in these locations.

Under a compromise deal, the monitoring equipment remains in the agency's custody but the data is in Iran's possession, and must not be erased as long as the arrangement remains in force.

Initially agreed for three months, the compromise was extended by another month and then expired on June 24.

The IAEA has since been urging Tehran to inform it of its intentions.

In its statement last Tuesday, the IAEA said its "verification and monitoring activities have been seriously undermined" by Tehran's actions.

It also said that Iran had boosted its stocks of uranium enriched above the levels allowed in the 2015 deal formally known as the Joint Comprehensive Plan of Action (JCPOA).

Iran's ultraconservative new President Ebrahim Raisi said Wednesday that his country was "transparent" about its nuclear activities.

The state-run Iran newspaper said the two sides would discuss Sunday "the temporary arrangement... on the supervision of IAEA inspectors and the contents of IAEA surveillance cameras installed in Iranian nuclear centres".

Grossi -- on his second trip to Iran this year -- was expected to hold a press conference on his return to Vienna airport around 8:30 pm (18:30 GMT).