BREAKING:At least 80 people were killed and 18 others were seriously injured Thursday when a terrorist drove a large truck loaded with guns and hand grenades into a crowd that had gathered for a Bastille Day fireworks display in the southern French city of Nice.

There was no immediate claim of responsibility, but two sources, including a U.S. counterterrorism source who collects and monitors jihadist social media, told Fox News that accounts linked to ISIS were “celebratory” and their followers were told to use the hashtag “Nice”.

The death toll was confirmed by French Interior Minister Bernard Cazeneuve early Friday morning. A source told Fox News that two Americans a father and his son were among the dead in the attack.

Earlier, French President Francois Hollande said that children were among the dead, and said his country was “under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing.”

Hollande also announced that he would extend France’s state of emergency by another three months, until Oct. 26. France has been on its highest state of alert since ISIS terrorists killed 130 people in Paris this past Nov. 13.

French Interior Ministry spokesman Pierre-Henry Brandet said the truck driver had been “neutralized”, or killed by police, and an investigation would confirm whether he had acted alone. The Paris prosecutor’s office announced that it was opening an anti-terrorism investigation into the attack.

Sky News and the Nice-Matin newspaper reported that the driver was a 31-year-old Nice resident of Tunisian origin. No other details of his identity were immediately avaiable.

A U.S. official told Fox News that the attack was in line with ISIS, which has become “increasingly brazen” in its attacks as it comes under increasing military pressure in Iraq and Syria.

A 2010 edition of Al Qaeda in the Arabian Peninsula’s propaganda magazine “Inspire” also called on would-be terrorists to “use a pickup truck as a mowing machine … to mow down the enemies of Allah.”

The horror began at around 10:30 p.m. local time. A local government official told Fox News that the truck was driving full speed when it hit the crowd on the Promenade des Anglais, a major boulevard along the Mediterranean Sea.

Damien Allemand, a journalist for Nice-Matin, said the fireworks display was over and people were getting up to leave when they heard a loud noise and screams.

“A fraction of a second later, an enormous white truck came along at a crazy speed, turning the wheel to mow down the maximum number of people,” he said.”I saw bodies flying like bowling pins along its route. Heard noises, cries that I will never forget.”

Allemand said people took shelter in a nearby restaurant, where he continued to hear people shouting for missing family members. He ventured out and saw bodies, blood and body parts all along the road.

“This evening, it was horror,” Allemand concluded.

Nice-Matin posted an image of the truck on Twitter, captioning it, “The truck that drove into the crowd.”

Le camion qui a fonc sur la foulepic.twitter.com/h4QuBabJMx

Nice-Matin (@Nice_Matin)July 14, 2016

Another witness, Wassim Bouhlel, told the Associated Press that he saw the truck drive into the crowd, then witnessed the driver emerge with a gun and start shooting.

“There was carnage on the road,” Bouhlel said. “Bodies everywhere.”

Video showed men and women — one or two pushing strollers — racing to get away from the scenes. And, in what appeared to be evidence of a gun battle, photos showed a truck with at least half a dozen bullet holes punched through its windshield.

Graphic footage showed a scene of horror up and down the Promenade, with broken bodies splayed out on the asphalt, some of them piled near one another, others bleeding out onto the roadway or twisted into unnatural shapes.

“Help my mother, please!” one person yells out amid a cacophony of screaming and crying. A pink girl’s bicycle is briefly seen overturned by the side of the road.

Nice’s public prosecutor told reporters early Friday that bodies of the victims were scattered for over a mile.

Another witness, identified only as Chloe, told FranceTV Info, “We heard gunfire, a lot. A crowd came to us and told us to run … We went into a tapas bar and hid in the bathroom for half an hour.

“We went out and again we heard people running and saying ‘truck, truck,’ so we hid in the bathroom for an hour. The manager took us out by the back door and then I went home.”

President Barack Obama said he condemned “what appears to be a horrific terrorist attack” in the strongest terms.

“On this Bastille Day,” Obama said, “we are reminded of the extraordinary resilience and democratic values that have made France an inspiration to the entire world, and we know that the character of the French Republic will endure long after this devastating and tragic loss of life.”

Secretary of State John Kerry called the tragedy “horrendous” and said it was “an attack against innocent people on a day that celebrates Liberty, Equality, and Fraternity,” a reference to France’s national motto. Kerry also asked U.S. citizens in Nice to directly contact friends and relatives to inform them of their well-being.

Presumptive Republican presidential nominee Donald Trump announced late Thursday that he would postpone the announcement of his running mate, which had been scheduled to take place Friday morning.

France’s ambassador to the United States, Gerard Araud, characterized the events in Nice as a “terrorist attack.”

“Our democracies — France, the United States, our other partners, we are besieged, we face a terrible threat,” Araud said at a Bastille Day reception at the French Embassy in Washington late Thursday.

July 14 is a national holiday in France that commemorates the storming of the Bastille prison in Paris and the start of the French Revolution.

Fox News’ Catherine Herridge, Lucas Tomlinson and The Associated Press contributed to this report.