'France is filled with sadness'

France has begun a period of national mourning for those killed and injured when a 31-year-old Tunisian ploughed a truck through a crowd watching Bastille Day fireworks in Nice.

At least 84 people were killed, including 10 children, and more than 200 were injured.

President Francois Hollande said the country would observe three days of mourning as he warned the death toll could rise further, with more than 50 people fighting for their lives following the attack in the resort city on Thursday night (Friday NZT).

Mr Hollande, who was due to chair crisis talks with his inner security cabinet, had already extended a state of emergency by three months.

"France is filled with sadness by this new tragedy," he said in a dawn address.

Meanwhile, authorities were investigating whether the attacker acted alone.

Prime Minister Manuel Valls said Tunisian-born Mohamed Lahouaiej-Bouhlel probably had links to radical Islam, but Interior Minister Bernard Cazeneuve cautioned it was too early to make that connection.

Anti-terrorism prosecutor Francois Molins said the 31-year-old was "completely unknown" to the intelligence services but the assault was "exactly in line with" calls from jihadist groups to kill.

Lahouaiej-Bouhlel, who was shot dead by officers at the scene, was known to police for petty crimes but was not on a watchlist of suspected militants. He had one criminal conviction for road rage, and was sentenced to probation three months ago for throwing a wooden pallet at another driver.

The dual-national delivery driver was originally from the northern Tunisian town of Msaken, about 10km outside the coastal city of Sousse. He had relatives who still lived there and visited Tunisia frequently, the last time eight months ago, security sources said.

He was identified by his fingerprints after his driver's licence, mobile phone and credit card were found inside the vehicle. He has three children but is separated from his wife.

The 19-tonne refrigerated lorry used in the attack was rented on 11 July in Saint-Laurent-du-Var, just to the west of Nice.

Lahouaiej-Bouhlel's ex-wife was in police custody, Mr Molins said. Police found one pistol and various fake weapons in his truck.

Latest in series of bloody attacks

The attack in Nice has plunged France again into grief and fear, just eight months after gunmen killed 130 people in Paris.

Those attacks, and one in Brussels four months ago, have shocked Western Europe, already anxious over security challenges from mass immigration, open borders and pockets of Islamist radicalism.

The truck zigzagged along the city's seafront Promenade des Anglais as a fireworks display marking the French national day ended on Thursday night.

It careered into families and friends listening to an orchestra or strolling above the Mediterranean beach toward the century-old Hotel Negresco.

Bystander Franck Sidoli said he had seen people go down before the truck finally stopped just 5m away from him.

"A woman was there, she lost her son. Her son was on the ground, bleeding," he told Reuters at the scene.

Dawn broke on Friday with pavements smeared with dried blood. Small areas were screened off and what appeared to be bodies covered in blankets were visible through the gaps.

The truck was still where it had come to rest, its windscreen riddled with bullets.

"I saw this enormous white truck go past at top speed," said Suzy Wargniez, a local woman, aged 65, who had watched from a cafe on the promenade. "It was shooting, shooting."

At Nice's Pasteur hospital, medical staff were treating large numbers of injuries. Waiting for friends who were being operated on was 20-year-old Fanny.

"The truck pushed me to the side. When I opened my eyes I saw faces I didn't know and started asking for help," she told Reuters. "Some of my friends were not so lucky. They are having operations as we speak."

Nice-Matin journalist Damien Allemand had been watching the firework display when the truck tore by. After taking cover in a cafe, he wrote on his paper's website of what he saw: "Bodies every five metres, limbs ... Blood. Groans."

"The beach attendants were first on the scene. They brought water for the injured and towels, which they placed on those for whom there was no more hope."

Security criticised

With presidential and parliamentary elections less than a year away, French opposition politicians seized on what they described as security failings that made it possible for the truck to career 2km through large crowds before it was finally halted.

Christian Ertosi, a security hardliner who was mayor of Nice until last month and is now president of the Riviera region in which Nice lies, had written on the eve of the attack to Hollande to demand more funding for police.

"As far as I'm concerned, I demand answers, and not the usual stuff," Estrosi said on BFM TV Friday morning hours after the attack, questioning whether the government provided enough national police officers for the fireworks display.

On social media, supporters of militant group Islamic State celebrated the high death toll and posted a series of images, one showing a beach purporting to be that of Nice with white stones arranged to read "IS is here to stay" in Arabic.

France is a major part of a US-led mission conducting air strikes and special forces operations against Islamic State, as well as training Iraqi government and Kurdish forces.

France has also sent troops to West Africa to battle Islamist insurgents.

After the Paris attacks, Islamic State said France and all nations following its path would remain at the top of its list of targets as long as they continued "their crusader campaign".

- Reuters