PARIS/NICE (AFP) - France has called on young "patriots" to join the reserves following an Islamic State-claimed truck attack in Nice which killed 84 and prompted tough questions over security failures.

Interior Minister Bernard Cazeneuve made the appeal for volunteers to boost the security service on Saturday as France reels from its third major Islamist attack in the past 18 months.

"I want to call on all French patriots who wish to do so, to join this operational reserve," said Cazeneuve of a force which is currently made up of 12,000 volunteers aged between 17 and 30.

The call came as the government faces searing criticism over the Thursday attack, in which Tunisian Mohamed Lahouaiej-Bouhlel ploughed a 19-tonne truck into a crowd of people who had been watching Bastille Day fireworks in the French Riviera city.

Some 300 people were injured in the assault and many remain in a critical condition.

Opposition politicians and newspapers demanded more than "the same old solemn declarations", and far-right National Front leader Marine Le Pen called on Cazeneuve to step down.

"In any other country in the world, a minister with a toll as horrendous as Bernard Cazeneuve - 250 dead in 18 months - would have quit," she said.

But Cazeneuve defended France's security efforts, saying the country was facing "a new kind of attack" which highlighted "the extreme difficulty of the anti-terrorism fight".

Speaking as France began three days of mourning, he said 31-year-old Lahouaiej-Bouhlel "had not been known to the intelligence services because he did not stand out... by being linked with radical Islamic ideology".

Lahouaiej-Bouhlel's neighbours and father earlier described him as a violent depressive who showed no outward signs of being a devout Muslim.

The Islamic State group claimed responsibility for the killings on Saturday, three days after the assault.

In a statement via its Amaq news service, ISIS said one of its "soldiers" carried out the attack "in response to calls to target nations of coalition states that are fighting (ISIS)".

After crisis talks in Paris, French Defence Minister Jean-Yves Le Drian noted that ISIS had recently repeated calls for supporters to "directly attack the French, Americans, wherever they are and by whatever means".

"Even when Daesh is not the organiser, Daesh breathes life into the terrorist spirit that we are fighting," he said, using an Arabic name for ISIS.

Police said they had arrested four more people linked to Lahouaiej-Bouhlel, in addition to his estranged wife who was taken into custody on Friday.

Cazeneuve said the father-of-three "seemed to have been radicalised very quickly, from what his friends and family" have told police.

"We are now confronted with individuals open to ISIS's message to engage in extremely violent actions without necessarily having been trained or having the weapons to carry out a mass (casualty) attack," he said.

ISIS also claimed responsibility for the Nov 13 attacks which killed 130 people in Paris, while gunmen in January 2015 attacks on the Charlie Hebdo weekly and a Jewish supermarket were linked to both ISIS and Al-Qaeda.

A French parliamentary inquiry last week criticised numerous failings by the intelligence services over the Paris massacres.

France is also home to hundreds of extremists who have flocked to fight alongside ISIS.

At least 10 children and adolescents were among the dead in Nice as well as tourists from the United States, Russia, Ukraine, Switzerland and Germany.

A spokeswoman for the city's paediatric hospital said 16 bodies had not yet been identified.

The first compensation payouts to victims under a French government scheme will begin next week, minister Juliette Meadel told AFP Saturday.

Five children and 21 adults remained in a critical condition and were among 121 people still hospitalised, the French health ministry said.

Tahar Mejri is one of 30,000 people who had gone to watch the fireworks on the palm tree-lined Promenade des Anglais when their night turned to horror as the truck left mangled bodies strewn in its wake.

He lost his wife in the attack and was hunting Saturday for his four-year-old son.

"I have called everywhere, police stations, hospitals, Facebook and I can't find my son. I have been looking for him for 48 hours," he told AFP.

"My wife is dead, where is my son?"

Hours later, he emerged wailing in distress from the Pasteur Hospital in the north of Nice after learning that his son was also dead.