French Prime Minister Manuel Valls says the man who killed at least 84 people by ploughing his truck through a crowd in Nice in all likelihood had ties to radical Islamist circles.
Interior Minister Bernard Cazeneuve was more cautious, saying that the attacker's motives were not yet known.
"He is a terrorist probably linked to radical Islam one way or another," Valls told France 2 television's evening news programme.
"Yes, it is a terrorist act, and we shall see what the links there are with terrorist organisations."
He said the toll from the attack could rise.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Cazeneuve told broadcaster TF1 that the attacker was not known to intelligence services and said he could not confirm his motives were linked to jihadism.
Valls said he was "convinced" that France would win the war against terrorism and radical Islam but warned that the fight would be protracted.
"As head of government, I owe the French people the truth. It will be long war outside and also inside our territory," Valls said.
He denied there were security failures and that the attack could have been prevented.
"We have given security forces all the means and resources they need," Valls said, adding that 15 attempted attacks have been prevented in the last three years.
"The same security setup last night was put in place by the state and local authorities during the Nice carnival in January and the Euro 2016 football tournament," he said.
