The Islamic State militant group has said it was behind the massacre in Nice in which at least 84 people were killed including 10 children.

The claim of responsibility, reported by the jihadist-linked Amaq news agency, comes as five people were detained following the attack in the French resort city.

Amaq quoted an IS security source as saying one of its "soldiers" carried out the atrocity "in response to calls to target nations of coalition states that are fighting (IS)".

Mohamed Bouhlel drove a 19-ton lorry into a crowd which had been watching a fireworks display on the seafront during Bastille Day celebrations on Thursday evening.

Sky's Foreign Affairs Editor Sam Kiley said IS' claim of responsibility should be treated with a "degree of caution".


Image: The lorry attack was carried out by Mohamed Bouhlel

Kiley said: "The statement refers to Bouhlel as some sort of soldier for the caliphate but they haven't prepared any propaganda ahead of time that would indicate this was part of a formed plot, which they have done so in the past - the martyr videos."

Sources said those arrested were members of the attacker's "close entourage".

Two men were reportedly held on Saturday in a raid near Nice's main train station and a third was detained at another address in the resort city.

The driver's ex-wife was being questioned on Friday.

Citing officials, Kiley said 62 national police were on duty for the whole of Nice on the day of the attack and 50 municipal police who were largely responsible for traffic control.

He said: "By anybody's standards that would be considered grossly inadequate and might explain why no barrier was put up on the promenade."

Kiley said the day before the atrocity, the president of the Regional Council of Provence-Alpes-Côte d'Azur, Christian Estrosi, wrote to President Francois Hollande, saying the police were grossly overworked, short-staffed and would not be able to protect Nice from a terror attack.

Witnesses Describe Attack In Nice

Of the more than 200 people injured in the massacre, 26 are still in a critical condition in hospital, 21 of them adults and five children.

The children are being treated at the Lenval Foundation paediatric hospital in Nice, where 30 youngsters were taken on the night of the attack.

A hospital spokesman said most were suffering from head trauma and fractures and one was just six months old.

Authorities have been trying to find out whether Bouhlel, a 31-year-old French citizen born in Tunisia, acted alone or with accomplices, and what his motives were.

His father Monthir Bouhlel said his son had undergone psychiatric treatment in the past.

"He had psychological problems that caused a nervous breakdown," he told the TF1 and France 2 television channels.

"He would become angry, shout, break everything around him. We had to take him to the doctor."

French Prime Minister Manuel Valls said Bouhel was a terrorist and one way or another had links to radical Islam.

Interior Minister Bernard Cazeneuve said he had been "radicalised very quickly" and the case demonstrated the "extreme difficulty of the fight against terrorism".

Bastille Day Attack: Why Has France Been Targeted?

France's chief prosecutor Francois Molins said Bouhlel had a record for petty crime and had been sentenced to six months in jail in March for a road rage incident a few months before.

But the sentence was suspended as it was his first.

The delivery driver and father of three "was totally unknown by the intelligence services and he had never been subject to any sign of radicalisation", Mr Molins said.

Bouhlel had driven more than a mile in the rented truck, zig-zagging across the road to run over as many people as possible, witnesses said.

Many fled into the sea to escape the attack, with others "flying like bowling pins" as Bouhlel went on the rampage.

Mr Molins told reporters Bouhlel had shot three police officers before he was killed and slumped onto the passenger seat.

President Hollande described the attack as a "monstrous terrorist act" and declared three days of national mourning starting on Saturday.

Meanwhile, hacking group Anonymous says it has vowed to track down members of the group behind the attack in Nice, with a spokesman saying "expect total mobilisation".