15 victims of Nice attack remain 'between life and death'

All 84 people killed in the Nice truck attack on Bastille Day have now been formally identified.

French officials added that the death toll has not increased though several people who were injured in the carnage remain in a critical condition.

Prosecutor's office spokeswoman Agnes Thibault-Lecuivre said some of the 84 French and foreign nationals who lost their lives in last Thursday's massacre died subsequently in hospital.

Authorities have not released an official list naming the victims.

In a speech to army reserves on Wednesday, French President Francois Hollande said 15 people injured in the attack are still in hospital "between life and death".

He praised the French emergency and security services' work during and after the attack in which Mohamed Lahouaiyej Bouhlel, a troubled 31-year-old Tunisian, drove a truck down a seaside boulevard full of people who had watched a fireworks display.

Officials say five unnamed people remain in custody for possible links to the attack and two of them have had their detentions extended exceptionally beyond the 96-hour deadline.

A sixth unnamed person detained over the attack has been released.