French police have arrested two more people in connection with an attack that killed at least 84 people when a lorry deliberately drove into crowds celebrating the country's main national holiday in Nice.

French judicial sources told the Reuters news agency on Sunday that a man and a woman with ties to the attack claimed by the Islamic State of Iraq and the Levant (ISIL, also know as ISIS) group were detained but provided no details on their identities. 

Five other people were already in custody, including the estranged wife of Mohamed Lahouaiej-Bouhlel, a Tunisian with no apparent links to ISIL. Bouhlel was shot dead by police.

Authorities are trying to determine whether Bouhlel, a 31-year-old who had lived in Nice for many years, was acting alone in Thursday's attack.

ISIL said it carried out the attack in response to calls to target civilians in countries that are part of a coalition fighting ISIL.

Speaking from Nice on Friday, French President Francois Hollande said about 50 people were in a critical condition, still "between life and death" after the attack.

The attack also injured more than 200 people, including many children, and left the area strewn with bodies.

France declared three days of national mourning after the Bastille Day massacre.

Source: Agencies