The protests in Iran are the most serious display of unrest in the country since the current regime came to power in 1979, a senior US official said on Friday.

US Assistant Secretary of State for Near East Affairs Barbara Leaf said the demonstrations that began in mid-September present a serious challenge to the regime.

“It's certainly by any judgment the most serious unrest of an organic nature that the regime has seen since the revolution,” Ms Leaf said during an event at the Woodrow Wilson Centre in Washington.

Protests erupted across Iran following the death of 22-year-old Mahsa Amini, who died in police custody. In confronting the protests, Tehran has resorted to violence, intimidation and mass arrests but has not been able to silence dissent.

She assessed that “the regime has a very difficult time understanding or contending with [the protests]".

Human rights organisations have estimated that more than 200 protesters have been killed in the crackdown by security forces and hundreds have been detained.

But Ms Leaf said that the worst may still be to come.

“The Iranian regime has not turned on the worst of its security forces yet on the public — it can still do worse,” she said.

The US official reiterated that Washington’s attention is solely focused on the protests and not on reviving the Iran nuclear deal.

“Our focus is on calling out and supporting [and] lifting up the voices of Iranian women and others who support them in their quest,” she said.

Talks over the so-called Joint Comprehensive Plan of Action, signed between Iran and world powers in 2015 to place limits on Tehran's nuclear programme in exchange for sanctions relief, have stalled and western officials are not optimistic over the deal's future.

Asked about Lebanon and the power vacuum that has developed following the resignation of former president Michel Aoun, Ms Leaf said the administration of US President Joe Biden has prepared for a set of chaotic scenarios that could unfold.

“I can see a scenario where there is disintegration, that [would be] the worst, where there's just an unravelling,” she said.

Stopgap measures such as payments the US has made to the country's armed forces may not be enough in such an event.

“They lose control of things. There's mass migrations … and I somehow imagine a lot of the same parliamentarians packing their bags and going off to places in Europe or elsewhere where they have property,” she said.

But the US official framed that as the worst-case scenario and that there are many in between that could occur before the parliament agrees on a new president.

“Things will have to get worse before the public pressure mounts in such a way that they feel that we are putting pressure directly on political leaders to do their work,” she said.

Ms Leaf, who recently completed a trip to the Gulf, expected a bigger assisting role for Saudi Arabia in Lebanon to mitigate the crisis.

“At different intervals, the Saudis stepped back but I think they will step back in,” she said.

On the maritime deal between Lebanon and Israel, Ms Leaf did not expect Israel's new prime minister-designate Benjamin Netanyahu to withdraw from it, which he has threatened to do in the past.

“Hopefully”, she said, the new Israeli leader will not scupper the historic deal that defined the maritime border between the two countries, which are technically at war.