During his first term in the White House from 2017 to 2021, Trump was the architect of a policy called "maximum pressure" which reimposed sweeping economic sanctions that had been lifted under a landmark 2015 nuclear deal.

"Rafael Grossi... who arrived in Tehran last night at the head of a delegation to negotiate with the country's top nuclear and political officials, met with Foreign Minister Abbas Araghchi," Iran's official IRNA news agency reported.

Later, Grossi is expected to meet President Masoud Pezeshkian in their first meeting since his election earlier this year.

He is also scheduled to meet the head of Iran's Atomic Energy Organisation, Mohammad Eslami, before addressing a joint news conference.

Grossi's visit is his second to Tehran this year but his first since Trump's re-election.

In 2018, Trump unilaterally abandoned the 2015 deal that gave Iran relief from international sanctions in exchange for restrictions on its nuclear programme designed to prevent it developing a weapons capability, an ambition it has always denied

- Search for solutions -

The following year, Iran started to gradually roll back its commitments under the deal, which barred it from enriching uranium to above 3.65 percent purity.

The IAEA says Iran has significantly expanded its stocks of uranium enriched to 60 percent, a level that has triggered international alarm as it is much closer to the 90 percent level needed for a nuclear warhead.

The head of the IAEA "will do what he can to prevent the situation going from bad to worse" given the significant differences between Tehran and Western capitals, said Ali Vaez, an Iran specialist at the Crisis Group, a US-based think tank.

Iran has blamed the incoming US president for the standoff.

"The one who left the agreement was not Iran, it was America," government spokeswoman Fatemeh Mohajerani said on Wednesday.

"Mr. Trump once tried the path of maximum pressure and saw that this path did not work."

Grossi's visit comes just days after Israeli Defence Minister Israel Katz said Iran was "more exposed than ever to strikes on its nuclear facilities".

The archfoes have exchanged unprecedented direct attacks in recent months as tensions soared during the intensifying war between Israel and Iran allies Hamas in Gaza and Hezbollah in Lebanon.

Trump's looming return to the White House in January has only added to international fears of all-out conflict between Israel and Iran.

"The margins for manoeuvre are beginning to shrink," Grossi warned in an interview with AFP on Tuesday, adding that "it is imperative to find ways to reach diplomatic solutions".

- Religious decree -

Grossi has said that while Iran does not have any nuclear weapons at this moment in time, it does have plenty of enriched uranium that could eventually be used to make one.

Iran's new president, who won election in July on a platform to improve ties with the West, has said he wants to revive the 2015 nuclear deal.

But all efforts to get the nuclear agreement off life support have failed.

The IAEA chief has repeatedly called for more cooperation from Iran.

In recent years, Tehran has switched off surveillance devices used to monitor its nuclear programme and effectively barred IAEA inspectors.

The foundations of Iran's nuclear programme date back to the late 1950s, when the United States signed a civil cooperation agreement with the Western-backed shah, Mohammad Reza Pahlavi.

In 1970, Iran ratified the Non-Proliferation Treaty (NPT), which requires signatory states to declare and place their nuclear materials under IAEA control.

But with Iran threatening to hit back at Israel for its latest missile strikes, some lawmakers have called on the government to revise its nuclear doctrine to develop an atomic bomb.

They called on supreme leader Ayatollah Ali Khamenei, who wields ultimate authority in Iran, to reconsider his longstanding religious edict or fatwa banning nuclear weapons.