French forensics officers arrive at the site of a knife attack in Nice on Thursday. AFP

The knife-wielding attacker suspected of killing three people at a church in the southern French city of Nice on Thursday is a 21-year-old Tunisian migrant who arrived in Europe just a few weeks ago, sources close to the inquiry said.

Meanwhile, Tunisia has opened an investigation after reports that the suspect in the Nice attack is Tunisian, the spokesman for a specialised counter-militancy court, Mohsen Dali, said.

Forensics officers deploy stretchers at the site of a knife attack as soldiers stand guard the street in Nice. AFP

"The public prosecutor of the anti-terrorism court has opened a forensic investigation into the suspicion that a Tunisian committed a terrorist operation abroad," Dali said.

The suspect, identified as Brahim Aouissaoui, landed in late September on the Italian island of Lampedusa, where he was placed in virus quarantine by authorities before being released with an order to quit Italian territory. He arrived in France in early October, the sources said.