Rafael Grossi's visit comes amid fears that Iran, which has reneged on some of its commitments since the United States pulled out of a landmark nuclear deal in 2018, is getting closer to building an atomic bomb.

Iran denies any such ambitions.

Here is a timeline of key nuclear developments since the ultra-conservative Raisi came to power in 2021:

- August 2021: firm on sanctions -

On taking office in August 2021, Raisi says Tehran backs any diplomatic moves to lift crippling US sanctions on his country but will not bow to Western pressure.

International sanctions against Iran were eased after it agreed to limit its nuclear activities under the deal reached in 2015, but former US president Donald Trump reinstated them after pulling out of the accord.

- March 2022: new tough measures -

Washington announces new sanctions targeting several entities it says are involved in procuring supplies for Iran's ballistic missile programme.

Tehran says the move shows US "ill will".

- June 2022: UN condemnation -

For the first time in two years, the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), adopts a resolution submitted by Britain, France, Germany and the US condemning Iran.

Tehran responds by removing surveillance cameras at nuclear facilities.

- June-July 2022: talks stalemate -

On June 16, 2022, Washington sanctions a network of Iranian petrochemical firms, as well as alleged front companies in China and the United Arab Emirates.

Later the same month, EU-brokered indirect talks in Doha between Iran and the US conclude with "no progress made", the State Department says.

On a visit to Jerusalem in July 2022, US President Joe Biden vows to use "all" American power to stop Tehran from acquiring nuclear weapons.

A month later, the European Union's foreign policy chief Josep Borrell submits a draft text on saving the 2015 deal.

- September 2022: competing demands -

In September 2022, Raisi reiterates on his debut visit to the United Nations that Iran is not seeking an atomic weapon and demands "guarantees" that the US will not withdraw again from a revived deal.

Israel's then-prime minister Yair Lapid tells the UN two days later the international community should use military force against Iran if it develops nuclear weapons.

- October 2022: US citizen released -

Baquer Namazi, one of four US citizens being held in Iran whose release Washington had demanded as part of the nuclear talks, is freed.

Three weeks later Raisi accuses the US of "dragging their feet" in the negotiations.

- February 2023: 'worrisome pace' -

On February 2, 2023, the IAEA says Iran has made covert changes to equipment at its Fordo uranium enrichment plant.

CIA director William Burns says on February 26 that Iran's nuclear programme is advancing at a "worrisome pace".

Two days later the IAEA says in a confidential report seen by AFP it has detected particles of uranium enriched to 83.7 percent, just under the 90 percent needed to produce an atomic bomb.