France's anti-terrorism investigation department was asked to take over inquiries into the Thursday night attack in which dozens were killed by a truck that mowed through crowds of people in the southern coastal resort town of Nice, media reported.
In the immediate aftermath of an incident that initial reports said killed at least 80 people, officials declined to formally declare the attack as terrorist assault pending further news. But several French media said the anti-terrorist investigation department had been put on the case.
The French government denied there was a hostage-taking incident after the truck plowed into the crowd, which had been watching a Bastille Day fireworks display.
"There is no hostage-taking," Interior Ministry spokesman Pierre-Henry Brandet told AFP, denying dozens of rumors following the incident. "An individual drove a truck into the crowd. He was killed by police." He added that investigations were under way to determine whether any accomplices were involved.
The president of the region that includes Nice said the truck was loaded with arms and grenades. Christian Estrosi told BFM-TV that "the driver fired on the crowd, according to the police who killed him.''
Wassim Bouhlel, a Nice native interviewed by The Associated Press, said he witnessed the man emerge with a gun and start shooting, but Sylvie Toffin, a press officer with the local prefecture, said that "to my knowledge'' there was no gunfire.
In Washington, President Barack Obama condemned "what appears to be a horrific terrorist attack," a White House statement said.
The president said he had directed his team to offer any assistance France might need to investigate the attack.
In London, a Downing Street spokesman said Prime Minister Theresa May was "being kept updated on reports coming in from Nice. We are shocked and concerned by the scenes there. Our thoughts are with all those affected by this terrible incident on what was a day of national celebration. We stand ready to help any British nationals and to support our French partners."
A Foreign Office spokesman urged Britons in the Nice area to "follow the instructions of the French authorities."
This report contains information from AP, AFP and Reuters.