Soldiers, police officers and firefighters walk near dead bodies covered with a blue sheets on the Promenade des Anglais seafront in the French Riviera town of Nice on July 15, 2016, after a truck drove into a crowd watching a fireworks display.
"The attack was "undeniably" of a "terrorist nature".
The Nice incident was the third major attack in France in 19 months, a spate that includes the shootings at the office of the Charlie Hebdo magazine in Paris in January 2015 and coordinated attacks around the capital in November 2015.
Mobile phone footage shows police surrounding the lorry, firing their weapons at the driver.
"We owe our freedom to each other", Obama said, adding that the "entire world... stands in solidarity with the people of France during this hard time".
Tunisian man Mohamed Lahouaiej Bouhlel killed 84 people when he drove a truck into a Bastille Day crowd in Nice. But neighbors in the Nice neighborhood where the Bouhlel used to live told The Associated Press that his estranged wife had been taken away Friday by police.
At least three people were detained early on Saturday in the French city, Reuters reported police sources had said. He was married with three children, and was not known by the Tunisian authorities to hold radical or Islamist views.
In an address to the nation, French President Francois Hollande said the attack was done "to satisfy the cruelty of an individual and maybe a group".
"Our thoughts are with the people of France today". By morning, no group had officially claimed responsibility for the attack.
U.S. Democratic presidential candidate Hillary Clinton said the attack made her "sick at heart".
These were his first on-camera remarks since the terror attack in Nice on Thursday evening that has left 84 people dead and some 50 people "between life and death".
People also gathered in Melbourne where Victorian parliament and the Star Observation wheel turned the colours of the French tricolour to honour those impacted by the shocking event.
"Anyway the anti-terrorism prosecutor Fran?ois Moulin said it very clearly, yes, it's a terrorist action and we will see who are the accomplices and possible connections with terrorist organisations", Valls said.