A sunflower is seen on a fence in front of the Negresco hotel, on the Promenade Des Anglais, in memory of victims the day after a truck ran into a crowd at high speed killing scores and injuring more who were celebrating the Bastille Day national holiday, in Nice, France July 15, 2016. REUTERS/Eric Gaillard

UPDATE: 5:15 a.m. EDT — Islamic State group, also known as ISIS, claimed responsibility on Saturday for Thursday's attack in the French resort city of Nice, Reuters reported, citing a news agency that supports the extremist group.

"The person who carried out the operation in Nice, France, to run down people was one of the soldiers of Islamic State," Amaq news agency, which ISIS regularly uses to issue reports, said. "He carried out the operation in response to calls to target nationals of states that are part of the coalition fighting Islamic State."

UPDATE: 4:14 a.m. EDT — French police detained three people in Nice early on Saturday in connection with the investigation into Thursday’s attack, when a truck ploughed through a crowd and killed at least 84 people. The arrests were made in two different areas of Nice, Reuters reported.

Meanwhile, French authorities are yet to determine whether the 31-year-old Tunisian driver, identified as Mohamed Lahouaiej Bouhlel, carried out the attack with accomplices, and whether he was influenced by radical Islam.

Original Story:

French President François Hollande will chair a meeting with his inner security cabinet to discuss the response to Thursday's attack in the French resort city of Nice, when a truck loaded with weapons ploughed through a crowd and killed at least 84 people.

Hollande, who has already extended a state of emergency by three months, is expected to assess all available options in response to the attack, which also injured more than 200 people, of whom 52 are critical. Describing the attack as an act of terror, Hollande earlier said that operational reserves had been called up to assist police and security forces across the country.

“France was struck on the day of its national holiday, July 14, the symbol of liberty,” Hollande said. “France as a whole is under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing.”

The man, who was driving the truck, was identified as Mohamed Lahouaiej Bouhlel, a 31-year-old Nice resident of Tunisian descent. Bouhlel was later shot dead by two police officers inside the truck’s cabin.

Meanwhile, Hollande’s political rivals criticized the beleaguered French leader Friday over the deadly attack and accused authorities of not taking sufficient actions to prevent terrorism, the Wall Street Journal reported.

France has been under a state of emergency since the Paris attacks in November — carried out by the militants of the Islamic State group, also known as ISIS — killed 130 people. Following the attack, Hollande passed a law giving more power to investigators and created a homeland security force of 10,000 personnel.

However, opposition leaders said Friday that the Nice attack shows those moves by Hollande were not sufficient to counter terrorism.

“I’m not an investigator, but if the necessary measures had been taken, the catastrophe wouldn't have happened,” the Journal quoted Alain Juppé, a former prime minister and the current mayor of Bordeaux, as saying. “Of course more must be done, and better.”

While visiting victims in Nice on Friday, Hollande answered his critics and called for “unity and cohesion.”

“I won’t stoop to anything outrageous or excessive when it is a question of responding and responding justly to the challenges we face,” Hollande said.