Amnesty International has called on Iran to carry out an "impartial and transparent" investigation into the "suspicious" death in custody of prisoner Shahin Naseri, who claimed that he witnessed the torture of champion wrestler Navid Afkari before his execution on homicide charges.

The London-based rights group said it has received information that a day before his death in custody, Naseri called at least two people from the prison clinic using another prisoner's phone, expressed fears about his life being in danger, and indicated he'd been transferred from his regular ward to a cell intended for solitary confinement, but at the time used to detain several prisoners.

Iranian authorities said last month that they were investigating Naseri's death after the Oslo-based Iran Human Rights group quoted unnamed sources as saying Naseri had died under "suspicious circumstances" in the Greater Tehran Penitentiary after being transferred to solitary confinement.

The rights group said the source told it that Naseri had been transferred to an unknown location on the anniversary of the execution of Afkari, who had been convicted of murdering a security guard during mass anti-government protests in 2018.

Afkari was hanged on September 12, 2020, despite an international outcry after he said he was tortured into making a false confession, while his attorney said there was no proof of his guilt.

Iran's judiciary dismissed the torture claim.

Naseri claimed he witnessed Afkari being subjected to a severe beating by two plainclothes agents and that he had heard him scream.

A statement by the General Directorate Of Prisons of Tehran Province on September 23 said Naseri died 45 minutes after being taken to a prison clinic, where resuscitation efforts, including cardiac massage and artificial respiration, failed to save him.

The statement, which said Naseri was serving a prison term for fraud, theft, and forgery, added that the cause of his death was being investigated and would be announced later.

In a report released in September, Amnesty International said it had recorded at least 72 such deaths in custody since January 2010 despite credible reports that the deaths were the result of torture or other ill-treatment, or the lethal use of firearms and tear gas by officials.

The rights group said not a single Iranian official had been held accountable for these deaths.