Ignoring Israeli assassination threats and the warnings of US President Donald Trump that he must be involved in selecting Iran's new leader, the Assembly of Experts moved to declare Khamenei supreme leader just after midnight on Monday.

"Appointing Mojtaba Khamenei sends one clear message abroad: the system is closing ranks, not collapsing," said Ali Vaez, of the International Crisis Group (ICG) think tank.

Clement Therme, of the International Institute for Iranian Studies, said it was a symbolic choice, adding that conserving the Khamenei name was "very important for the propaganda of the regime".

The new supreme leader, aged 56, embodies continuity for the Islamic republic, despite his father previously rejecting the kind of hereditary leadership brought to an end in Iran by the Islamic revolution of 1979.

"Hatred of Israel and of the United States is part of the DNA, of the ideology of the regime," said Therme, and was not tied to any one individual.

Considered a conservative in the vein of his late father, Mojtaba Khamenei has close ties to the new chief of the Revolutionary Guards, Ahmad Vahidi.

Khamenei's elevation showed that "it's the hardest of hardliners in the Iranian system" who were now in power, said Therme.

Having survived domestically via "fear and violence", he added, they were now using violence abroad to prolong their rule.

- 'Pressure backfires' -

The Iranian authorities cracked down fiercely on anti-government protests in January, with rights groups documenting at least 7,000 deaths and warning the toll could be much higher.

The war with Israel and the United States, in the minds of Iran's hardliners, is the convergence of the fights against internal and external enemies, Therme said.

Both Israel and the US have talked of Iranians using the war as an opportunity to overthrow their government.

Religiously speaking, Mojtaba Khamenei was not an obvious candidate, suggested Bernard Hourcade, an Iran specialist at the CNRS in Paris.

The new supreme leader is "an ordinary mullah" who "does not have an uncontestable religious legitimacy", he said.

That he was chosen anyway showed that the Revolutionary Guards, the ideological arm of Iran's military and orchestrators of the January crackdown, had won a victory, said Hourcade.

Vaez, of the ICG, said the choice was a message to Washington "that all the economic, diplomatic and military pressure that the US has brought to bear on Iran has only replaced one Khamenei with another.

"It's a demonstration that pressure backfires."

- War blocks political change -

Continuing to fight the war with the United States and Israel has become an existential matter for Iran's leaders, said Hourcade.

Ultraconservatives, whose grip was weakened by the January protests, can argue they are merely defending the homeland from US-Israeli aggression.

For now, "the war completely blocks any possibility of political change in Iran", he added.

Therme, meanwhile, said it was too early to know what impact the shock of the war would have on the protest movement within Iran, but pointed out that the recent wave of demonstrations came six months after the previous war with the US and Israel in June last year.

Despite some public shows of support for the new supreme leader, "what we do know is that there is no rallying behind him", said Therme.

Nevertheless, it will be clear to Trump that "he will not be able to play one ayatollah against another".