With his dynamic manner and high-risk visit to Ukraine's Zaporizhzhia nuclear power station, International Atomic Energy Agency (IAEA) Director General Rafael Grossi has been at the centre of recent coverage of Russia's invasion.

After weeks of intensive negotiations to secure the visit, he set off for Zaporizhzhia early on Monday morning.

The photo he tweeted announcing his visit could have been a poster for an action film, with 61-year-old Grossi wearing a steely expression in front of serried ranks of IAEA experts.

The Zaporizhzhia site has been occupied by the Russians since early on in the conflict and fears of a nuclear catastrophe have been raised on several occasions by bombardments in the vicinity.

Despite fresh shelling in the area earlier this week, a flak-jacketed Grossi insisted that "we are not stopping" with the visit.

"Wish us luck," he told reporters at the plant, where he stayed for several hours.

The visit was criticised by Ukrainian President Volodymyr Zelensky, who said the IAEA had failed to push for the demilitarisation of the site and had not ensured access for independent media.

The Kremlin meanwhile called the visit "very positive".

Grossi had previously ventured to Ukraine in March and then again a month later to go to Chernobyl, the scene of the world's worst nuclear catastrophe in 1986.

Grossi will be glad to have established a permanent IAEA presence on site, with two inspectors expected to stay on there.

"Let the world know that the IAEA is staying at Zaporizhzhia," he said after the visit, adding that his team had "a lot more to do" and would seek to provide impartial updates on the situation there.

Western diplomats have lauded Grossi for undertaking the mission.

"He explores all avenues, it's very brave and I'm not sure another IAEA director general would have done the same," a European source said.

Senior French diplomat Philippe Errera tweeted his praise for Grossi's "perseverance".

In a comment sent to AFP, the American ambassador to international organisations in Vienna Laura Holgate said the US was "extremely grateful to DG Grossi and his team for their visit", calling it "a critical step in addressing nuclear safety" at the plant.

Kelsey Davenport from the Arms Control Association think-tank said that while "an on-site IAEA presence does not guarantee that a nuclear disaster will be averted... it should mitigate that risk and deescalate tensions".

Davenport said the "IAEA is facing unprecedented challenges" which constitute "a direct affront to the nuclear order".

"The agency needs leadership that will speak truth to power," she said, adding: "Grossi is proving to be that leader."

One of Grossi's current challenges is navigating the IAEA's role in the supremely complex and sensitive Iranian nuclear dossier.

After Iran and world powers struck deal on the country's nuclear programme in 2015, it fell to the IAEA to monitor Tehran's compliance with the deal's restrictions on its nuclear activity.

But since former US President Donald Trump pulled the US out of the deal in 2018, it has been gradually disintegrating and Iran has progressively abandoned those restrictions, as well as curbing access for IAEA inspectors to sites and data.

Grossi has warned that the agency risks "flying blind" in Iran, and has made several short-notice visits to Tehran to negotiate with senior Iranian officials.

While protracted diplomatic negotiations have been taking place to try to revive the 2015 accord, Iran has said an outstanding IAEA probe has to be concluded as part of the deal.

That investigation concerns the previous discovery of traces of enriched uranium at three sites not declared by Tehran.

Grossi has refused to back down, insisting that Iran has failed to adequately explain the presence of that material.

He has been head of the IAEA since 2019 and had previously served in high-ranking positions at the agency between 2010 and 2013.

The polyglot Argentinian father-of-eight is also rumoured to be in the running for the post of UN Secretary General when the incumbent Antonio Guterres' term expires in 2026.

Asked about this at a conference in New York last month, Grossi replied that "my plate is full of terribly important matters -- that is occupying my time and waking me up in the night, and this is all I care for".