Eighty-four people were killed after a truck ploughed through crowds of people celebrating Bastille Day in the southern French city of Nice.

Here is what we know so far about the attack that the Islamic State group claims was carried out by one of its “soldiers.”

How did the attack unfold?

A white 19-tonne rented refrigerator truck slammed into the crowd on Nice's seafront around 10:45 pm (2045 GMT) Thursday.

Some 30,000 people had gathered on the glitzy beachfront avenue, the Promenade des Anglais, to watch a firework show for France's national day.

The truck drove two kilometres (1.3 miles) through the crowd, Interior Minister Bernard Cazeneuve told reporters.

Anti-terror prosecutor Francois Molins said the driver fired “several times” on three police officers as they tried to bring the truck to a halt, and eventually shot him dead.

Details of the assault

The truck pulled onto a section of roadway that was open to traffic despite the fireworks. Agents watching via the city's network of surveillance cameras quickly spotted the vehicle because heavy trucks are not allowed in the area.

Though the pedestrian area was protected by metal barriers and police cars, the truck “forced its way onto the footpath” according to authorities.

Travelling at 90 kilometres per hour (55 mph), the truck blasted through the obstacles.

In order to stop the truck “you would have needed a concrete wall”, said an official. City Hall authorities noted that barely 45 seconds later the attacker was shot dead by police, and the carnage halted.

Who was the assailant?

Molins named the driver as a 31-year-old Tunisian, Mohamed Lahouaiej-Bouhlel, who lived in Nice, and was described by neighbours as a loner who never responded to their greetings.

His identity papers were found in the truck, along with a pistol and ammunition and a number of fake weapons including two replica assault rifles.

His body was found on the passenger seat, said the prosecutor.

Molins said Lahouaiej-Bouhlel had had various brushes with the law since

2010 for making threats, theft and violence, including a conviction in March this year for which he was given a six-month suspended sentence.

But he was “totally unknown” to the intelligence services, said Molins.

His estranged wife was held for questioning. Four other men believed to be linked to him have also been detained, a judicial source said Saturday.

The attacker's father, who lives in Msaken, eastern Tunisia, said his son had suffered from depression and had “no links” to religion.

IS claims attack

The Islamic State group claimed on Saturday that one of its “soldiers” carried out the attack “in response to calls to target nations of coalition states that are fighting (IS).”

The probe is being handled by anti-terrorism investigators.

The attack comes with France under a state of emergency following the Islamic State attacks in Paris in November that left 130 people dead.

Government response

Hollande announced that the state of emergency -- which he had said only the day before would end on July 26 -- will be extended by three months. Army reservists will be called up to boost security.

He also said France would strengthen its role in Iraq and Syria, where it is part of the international coalition fighting IS jihadists.

He chaired a meeting of top military and security officials later Friday to decide on possible further steps.

Has anything like this happened in France before?

In December 2014, two men ploughed their vehicles into pedestrians in separate incidents.

The first driver shouted “Allahu Akbar” (God is greatest) as he drove into people in the eastern city of Dijon, injuring 13.

The 40-year-old had a long history of mental illness, and no ties to jihadist groups, the government said.

A day later, a man rammed a white van into a Christmas market in the western city of Nantes, killing one person and injuring nine others. He then stabbed himself several times.

Prosecutors said a notebook was found in his vehicle in which he spoke of his “hatred for society” and said he feared “being killed by secret agents.”

The man committed suicide in his prison cell in 2016 while awaiting trial.

Who are the victims?

Eighty-four people were killed, including 10 children and adolescents, and around 300 sought treatment at hospitals.

Five children and 21 adults are still fighting for their lives in hospital, the health ministry said.

Hollande said “many foreigners and young children” were among those killed or injured.

The dead included nationals of Tunisia, France, Germany, the United States, Switzerland, Algeria, Poland, Morocco, Russia, Armenia and Madagascar.  (AFP)