A French anti-terrorist judge has charged three more men suspected of helping to arm the Islamist radical who crushed 86 people to death with a truck in Nice, a judicial source said Saturday.

President Francois Hollande invoked the spirit of national unity on Saturday as he led tributes to 86 people killed in a jihadist truck attack in Nice on Bastille Day.

Pope Francis on Saturday called for a "sincere dialogue" between Christians and Muslims as he met grieving relatives and survivors of France's Bastille Day attack, in which a jihadist ploughed his truck into a crowd.

French President Francois Hollande on Friday defended his government from accusations it relaxed its anti-terror defences after the Euro championships, announcing further support for the fight against the Islamic State group.

French Prime Minister Manuel Valls was booed in Nice on Monday before and after the minute of silence for the victims of the deadly Bastille Day attack.

The Islamic State group claimed responsibility for the truck attack that killed 84 people in Nice on France's national holiday, a news service affiliated with the jihadists said Saturday.

US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov on Friday offered their condolences at the French embassy in Moscow over the Nice truck attack that left at least 84 people dead.

French President Francois Hollande said Friday that "many foreigners and young children" were among those killed or injured after the truck attack on a crowd celebrating Bastille Day in Nice, with around 50 fighting for their lives.

An eight-month-old baby boy lost in the chaos after a truck rammed into a crowd in the French Riviera city of Nice killing at least 84 people was tracked down through Facebook on Friday.

Leading Muslim clerics joined Arab leaders on Friday in condemning a truck attack that killed at least 84 revellers in the Mediterranean resort of Nice on France's national holiday.

Politicians and public figures from around the world have reacted with horror after a truck smashed into a crowd in the French resort of Nice, killing at least 84 as they watched a Bastille Day fireworks display.