Bagheri, 56, had served as deputy to the foreign minister of the Islamic republic, Hossein Amir-Abdollahian, who perished along with the Iranian president Ebrahim Raisi and seven others in the accident.

Bagheri is considered close to Iran's ultraconservatives and a member of the inner circle of supreme leader Ayatollah Ali Khamenei, who is the father-in-law of Bagheri's brother.

The negotiator with the salt-and-pepper beard came to be known to his interlocutors for a soft-spoken composure that often stood in sharp contrast to his uncompromising stance.

"Every time foreigners intervene, under any pretext... their presence interferes with the security and stability of the region," Bagheri once said.

The new acting foreign minister has long experience with Iran's nuclear dossier -- the flashpoint issue that has pitted Tehran against major powers and arch foe Israel, who suspect it is seeking the atomic bomb.

Iran struck a landmark nuclear deal in 2015 in Vienna under which it agreed to curb the programme in return for sanctions relief, but former US president Donald Trump withdrew from it three years later.

After Iran had agreed the Vienna deal under its moderate president Hassan Rouhani, Bagheri emerged as one of its main critics, accusing the administration of bowing to the West.

He charged that the deal violated most of Khamenei's "red lines" by imposing curbs and strict surveillance on the nuclear programme, which Iran says is strictly for civilian purposes.

After the US withdrawal and reimposition of sanctions in 2018, Bagheri accused Rouhani of having "given the West the impression that Iran is very weak".

He later also used tough language to defend Iran's efforts to revive the deal.

Last year he charged that the critics of those efforts "in reality want to deprive the Islamic republic of a key and important tool to guarantee national interests".

Born in 1967 in the village of Kan in northwestern Tehran, Bagheri grew up in a family deeply involved in politics in the Islamic republic.

His father, a renowned Shiite cleric, was a member of the Assembly of Experts tasked with selecting the supreme leader. Bagheri's uncle, Mohammad Reza Mahdavi Kani, later led the same body.

Bagheri worked in the foreign ministry in the 1990s and grew close to conservative figure Saeed Jalili.

When Jalili was appointed secretary of the Supreme National Security Council and became Iran's chief nuclear negotiator, Bagheri served as his number two.

After Bagheri concluded his own mission at the Council, he joined the judiciary when it was headed by Raisi. He served as secretary of the human rights council and then assistant for international affairs.

Weeks after Raisi took office in 2021, Bagheri was appointed deputy foreign minister for political affairs and chief nuclear negotiator.

But those talks have since come to a standstill over seemingly insurmountable differences, particularly with Washington.