The visit comes at a time of heightened regional tensions and with the International Atomic Energy Agency (IAEA) criticising Iran for lack of cooperation on inspections and other outstanding issues.

Grossi, the head of the IAEA, "arrived in Tehran on Monday noon at the head of a delegation to participate in the nuclear conference and negotiate with top nuclear and political officials of the country," according to Tasnim news agency.

Other media in Iran confirmed his arrival.

He is scheduled to meet Iran's foreign minister Hossein Amir-Abdollahian as well as the Islamic republic's nuclear chief Mohammad Eslami.

Grossi is also expected to deliver a speech at Iran's first International Conference on Nuclear Science and Technology.

The three-day event, which starts on Monday, is being held in Isfahan province, home to the Natanz uranium enrichment plant and where strikes attributed to Israel hit last month.

The IAEA and Iranian officials reported "no damage" to nuclear facilities after the reported attack on Isfahan, widely seen as Israel's response to Iran's first-ever direct attack on its arch foe days earlier, which itself was a retaliation for a deadly strike on Tehran's Damascus consulate.

On Wednesday Eslami, head of the Atomic Energy Organization of Iran, said he was "sure that these negotiations will further help clear ambiguities, and we will be able to strengthen our relations with the agency".

Iran in recent years has deactivated IAEA monitoring devices at nuclear facilities and barred inspectors, according to the UN agency.

Grossi last visited Iran in March 2023 and met with top officials including President Ebrahim Raisi.

- 'Opportunity to share concerns' -

Iran has suspended its compliance with caps on nuclear activities set by a landmark 2015 deal with major powers after the United States in 2018 unilaterally withdrew from the agreement and reimposed sweeping sanctions.

Tensions between Iran and the IAEA have repeatedly flared since the deal fell apart, while EU-mediated efforts have so far failed both to bring Washington back on board and to get Tehran to again comply with the terms of the accord.

Last year, Iran slowed down the pace of its uranium enrichment, which was seen as a goodwill gesture while informal talks began with the United States.

But the Vienna-based UN nuclear agency said Iran accelerated the production of 60-percent enriched uranium in late 2023.

Enrichment levels of around 90 percent are required for military use.

Tehran has consistently denied any ambition to develop nuclear weapons, insisting that its atomic activities were entirely peaceful.

In February, the IAEA said in a confidential report seen by AFP that Iran's estimated stockpile of enriched uranium had reached 27 times the limit set out in the 2015 accord.

On Sunday, the Iranian official news agency IRNA said Grossi's visit provided "an opportunity for the two sides to share their concerns", especially with regard to the IAEA's inspectors.

Iran in September withdrew the accreditation of several inspectors, a move described at the time by the UN agency as "extreme and unjustified".

Tehran, however, said its decision was a consequence of "political abuses" by the United States, France, Germany and Britain.

Eslami said the IAEA has "more than 130 inspectors" working in Iran, insisting Tehran remains committed to cooperating with the nuclear watchdog.