A statement in the IS group affiliated Amaq news agency said one of its “soldiers” drove the heavy duty truck into crowds celebrating Bastille Day on Thursday night, BFMTV reported.
“He did the attack in response to calls to target the citizens of the coalition that is fighting the IS,” said the announcement by the jihadist outfit.
Lahouaiej-Bouhlel was known to the police as a petty criminal but was unknown to intelligence services, the prosecutor said. He was not known to be a radicalized Muslim.
But Prime Minister Manuel Valls said the man was linked to radical Islam and Paris prosecutor Francois Molins said the attack bore the hallmarks of jihadist terrorism.
French authorities meanwhile said five people believed to be linked to the truck driver were in police custody, including his estranged wife.
Three arrests were made on Saturday and two on Friday, Le Monde reported.
The man who drove the truck through thousands of people massed at the seafront has been identified as Mohamed Lahouaiej-Bouhlel, 31. He was shot dead by police.
French President Francois Hollande, who has called the attack “an undeniable terrorist nature”, has moved to extend a state of emergency by three months.
Prosecutors said Lahouaiej-Bouhlel, a Tunisian, drove the truck for about two kilometres along the promenade targeting unsuspecting people who had been enjoying fireworks and music.
BBC reported that of the 84 dead, 10 were children. Another 202 people were injured, with 52 of them in critical condition. Twenty-five were said to be on life support.
Some 30,000 people were on the Promenade des Anglais at the time of the attack.
Among the dead were residents of Nice and foreign tourists, among them three Algerians, a teacher and two schoolchildren from Germany, three from Tunisia, two Swiss, two Americans, a Ukrainian, an Armenian and a Russian.
Sixteen of the dead remained unidentified, BFMTV said.
BBC said there was a visible security presence in Nice on Saturday morning. Soldiers patrolled the front of the main train station in Nice.