- Who is Rafael Grossi? -

Grossi had been serving as Argentina's ambassador to the IAEA and is the agency's first leader from Latin America.

He previously held high-level posts at the agency between 2010 and 2013, bringing him into contact with Iranian officials at a time when international negotiations over Iran's nuclear activities were intensifying.

According to the former French ambassador to Iran Francois Nicoullaud, Grossi will able to draw on "solid experience in proliferation matters".

"He is someone of a very high calibre who comes from an important country in the nuclear field," says Nicoullaud.

A current Vienna-based diplomat said Grossi was expected to bring "a lot of energy and innovation" to the post, with a particular focus on pushing gender parity within the agency and promoting the role of nuclear energy in fighting climate change.

Grossi will be taking over from Yukiya Amano, who died in July at the age of 72 having been in post since 2009.

While the US is thought to have lobbied in his favour, diplomats say Grossi has stressed his belief in the importance of the agency's impartiality.

- Iran crisis looms -

Grossi begins his tenure amid a growing crisis over Iran's nuclear programme.

The IAEA is charged with monitoring the implementation of the 2015 Iran nuclear deal, which seems in danger of imminent collapse.

Without mentioning Iran by name, in his first speech to IAEA member states on Monday Grossi praised the agency's "impartial, accurate system of inspections".

"No one else can do what we can do, no one else can provide this credible assurance that nobody is diverting nuclear material to make nuclear weapons," he said.

Diplomats from Iran and the other remaining parties to the deal -- the UK, France, Germany, China and Russia -- will gather in Vienna on Friday to discuss ways forward for the crumbling deal.

Since May, Iran has taken a series of measures breaking limits on its nuclear activities laid down in the deal, with another one likely in early January.

Iran insists it has the right to do this in retaliation for the US withdrawal from the deal in 2018 and reimposition of crippling sanctions.

Ali Vaez from the International Crisis Group says the deal could collapse in a matter of weeks.

"The Iranians are simply running out of measures that are easy to reverse and non-controversial," he says.

Vaez adds that a fresh Iranian move to breach the deal could finally push the European signatories to trigger the "dispute resolution mechanism" foreseen in the 2015 accord -- which in turn could lead to the automatic resumption of UN sanctions on Iran.

The return of those sanctions would mark the "death blow" of the agreement, according to Kelsey Davenport of the Arms Control Association.

In such a scenario, says Vaez, "we will have a major non-proliferation crisis on our hands in the sense that the Russians and the Chinese have already declared they would not recognise the return of (sanctions)".

Some expect that in this scenario Iran would leave the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).

- Undeclared site -

There have been other points of friction between Iran and the IAEA in recent months.

The agency has been asking Iran to provide explanations for the presence of uranium particles at an undeclared site it took samples from in the spring.

While the IAEA has not named the site in question, diplomatic sources have said the agency asked Iran about a site in the Turquzabad district of Tehran, where Israel has alleged secret atomic activity in the past.

Added to this is an incident in October in which an IAEA inspector was briefly prevented from leaving Iran after triggering a security alarm.

She was allowed to return to Vienna but the IAEA has condemned her treatment as "not acceptable". One Vienna-based diplomat said Tehran had received little support in the matter, "even from countries that normally do sympathise".