A member of the Assembly of Experts that selects Iran's supreme leader has been killed in an armed attack, authorities said, adding that the attacker has been placed in custody.

The attack on Wednesday is believed to be the most significant in years against a cleric in Iran.

"Abbas Ali Soleimani was killed this morning in an armed attack... the assailant was also arrested and is now being investigated," IRNA news agency reported, citing a security official for the northern province of Mazandaran where the killing occurred.

The attack took place inside a bank in Babolsar city, the official said.

"The motive of the assailant is not yet clear and will be announced after it is clarified," the official added.

The governor of Mazandaran, Mahmoud Hosseinipour, said the attacker was a local security officer of the bank.

"So far, our information and documents indicate that this was not a security or terrorist act," Hosseinipour told state television, adding that the assailant did not know the cleric.

CCTV camera footage released by Tasnim news agency showed the security guard, wearing a blue and white jacket, shooting the cleric from behind as he was sitting in a chair at the bank.

Following the killing, Iranian Prosecutor General Mohammad Jafar Montazeri ordered provincial authorities to "investigate the issue from different angles, do the necessary follow-ups and report the results."

Soleimani, 75, was previously a representative of the country's Supreme Leader Ali Khamenei.

He had also been the imam who led the weekly Friday prayers in the cities of Kashan, in Isfahan province, and Zahedan in the southeastern province of Sistan-Baluchistan.

Under the constitution, the 88-strong Assembly of Experts is mandated with supervising, dismissing and electing the Supreme Leader.

The all-powerful deliberative body is now headed by ultra-conservative 96-year-old cleric Ahmad Jannati.

Its members are chosen in popular elections for eight year-terms from a pool of candidates vetted by the country's Guardian Council.

Previous attacks on clerics

In April 2022, a suspected knife attack in the northeastern shrine city of Mashhad in Razavi Khorasan Province led to death of two clerics and injury of another.

The chief suspect, identified as Abdolatif Moradi, 21, who had entered Iran illegally via the Pakistani border a year earlier, Tasnim news agency said at the time.

Moradi was hanged in June in the same city on the accusation of "moharebeh", or "war against God".

The assailant struck on the third day of the holy month of Ramadan as large crowds of worshippers gathered at the shrine of Imam Reza, one of the most revered figures in Shia Islam.

The attack in Mashhad came days after two Sunni clerics were shot dead outside a seminary in the northern Iranian town of Gonbad-e Kavus.