France has urged its nationals to leave Iran as soon as possible, saying they are exposed to the risk of arbitrary detentions.

"Any French visitor, including dual nationals, is exposed to a high risk of arrest, arbitrary detention and unfair trial," the French foreign ministry said on its website.

France this week lashed out at Iran for "dictatorial practices" and taking two of its citizens hostage after a video aired yesterday in which they appeared to confess to spying, amid weeks of unrest that Tehran has tied to foreign foes.

The French foreign ministry earlier today called for Iran to release its two nationals.

It comes as Iran said an investigation into the death in custody of Mahsa Amini found she lost her life to illness rather than reported beatings that sparked three weeks of bloody protests.

Ms Amini was 22 years old when she died on 16 September, three days after falling into a coma following her arrest in Tehran by the morality police for allegedly breaching the Islamic republic's strict dress code for women.

Anger over her death has triggered the biggest wave of protests to rock Iran in almost three years and a crackdown that has killed dozens of protesters and seen scores arrested.

Despite security personnel using lethal force, the women-led protests have continued for 21 consecutive nights, according to online videos verified by AFP.

Anger over Mahsa Amini's death has triggered the biggest wave of protests to rock Iran in almost three years

Iran's forensic organisation said today that "Mahsa Amini's death was not caused by blows to the head and vital organs and limbs of the body".

The death of Ms Amini, whose Kurdish first name is Jhina, was related to "surgery for a brain tumour at the age of eight", it said in a statement.

Her bereaved parents have filed a complaint against the officers involved, and one of her cousins living in Iraq has said that she died of "a violent blow to the head".

Other young women and girls have been killed at the protests, but rights group Amnesty International says Iran has been forcing televised confessions out of their families to "absolve themselves of responsibility for their deaths".