President Barack Obama condemned what he said “appears to be a horrific terrorist attack” in Nice.
He added: “Our thoughts and prayers are with the families and other loved-ones of those killed.”
Noting that the attack occurred on Bastille Day, Mr Obama praised “the extraordinary resilience and democratic values that have made France an inspiration to the entire world”.
Secretary of state John Kerry called it a “horrendous attack in Nice. … I was proud to stand alongside French leaders earlier today at Bastille Day celebrations in Paris, and the United States will continue to stand firmly with the French people during this time of tragedy”.
Both presidential candidates also condemned the attacks, with Republican Donald Trump declaring “this is war” and Democrat Hillary Clinton vowing “we will not be intimidated”.
Mrs Clinton said: “Every American stands in strong solidarity with the people of France, and we say with one voice: we will not be intimidated. We will never allow terrorists to undermine the egalitarian and democratic values that underpin our very way of life.”
Mr Trump, who postponed plans to announce his vice-presidential pick because of the attacks, said: “This is war. If you look at it, this is war coming from all different parts. And frankly it’s war and we’re dealing with people without uniforms.”