Sean Copeland, 51, and his 11-year-old son, Brodie, were killed in front of their horrified relatives as the lorry ploughed into crowds.
The U.S. Consulate in Marseille advised U.S. citizens in Nice to call family and friends to notify them that they are safe.
This breaks my heart.
The truck's driver was killed by police after an exchange of gunfire. On vacation from Texas.
President Barack Obama condemned what he said "appears to be a horrific terrorist attack".
"Senseless acts like this one are not isolated events, and we will continue to work with our Allies and partners to fight terrorism in all of its forms".
President Francois Hollande has said the country's state of emergency would be extended for another three months.
Belgian Prime Minister Charles Michel said authorities had been aware of a scenario in which a vehicle could be used for a terrorist attack. "That's the worst of it, because we expect it at every time, everywhere", said another. France has been struck once again in her flesh.
News of the attack came too late in the evening for some French titles - Liberation led with stories on Boris Johnson and the war in Mali, although the attack was the top story on its website.
Trudeau added in a statement that Canada has offered "all possible assistance to the French government". We will not give them up. The attack also caused presumptive Republican presidential candidate Donald Trump to postpone Friday's planned announcement of his running mate, reported to be Indiana Gov. Mike Pence.
World leaders and organisations from around the world have reacted with horror and expressions of solidarity with France in the wake of the Nice attack.
In the past weeks, various attacks hit other countries, including Turkey's Istanbul Ataturk airport.
Forensic officers stands near a truck with its windscreen riddled with bullets, that plowed through a crowd of revelers who'd gathered to watch the fireworks in the French resort city of Nice, southern France, Friday, July 15, 2016. Other photos show bodies covered in tarps.
"France is afflicted, but she is strong, and she will always be stronger than the fanatics who want to strike her today", he said. "It was completely insane, the crowd panicked and everyone was trying to flee". "And I think people getting use to this kind of attack". The tables, the chairs, the glasses got turned over. "There was carnage on the road", Bouhlel said.
"A person jumped onto the truck to try to stop it", Ciotti told Europe 1 radio.