The Bastille Day attacker who killed at least 84 people in the French Riviera city of Nice had been planning the attack for months with the help of at least five accomplices, the Paris prosecutor said on Thursday.

"The investigation underway since the night of July 14 has progressed and not only confirmed the murderous premeditated nature of Mohamed Lahouaiej Bouhlel's act but also established that he benefited from support and complicity," Francois Molins told a news conference.