The truck used in the deadly attack is riddled with bullet holes after the man identified as a French-Tunisian drove it through a crowd at high speed, killing scores of people celebrating Bastille Day in Nice. [Photo/Agencies]

Belgium held a meeting of officials and said later that there were fears of a second such attack using a vehicle on Belgian soil.

In the United Kingdom, newly appointed Prime Minister Theresa May said Britain stood "shoulder to shoulder" with France against what she called "these murderers".

The BBC reported that UK intelligence officers were working with their French counterparts investigating the attack.

An extra 3,000 police and army reservists were being drafted in France to add to the 7,000 already on terror attack duty throughout the country.

Hollande said he was extending by three months the state of emergency first imposed after the gun and bomb attacks in Paris in November on cafes, restaurants, a soccer stadium and a theater that killed 130 and injured more than 360.

French Prime Minister Valls said: "Terrorism is a threat that is weighing heavily on France.

"We are faced with a war that terrorism has brought against us. The goal of the terrorists is to make us scared. We won't give in to the terrorist threat. We must stand together united. Times have changed, and we should learn to live with terrorism."

Alain Juillet, former intelligence chief with France's Directorate of External Security, said: "We can see the possibility of more attacks by individuals.

"For France, it is almost impossible to stop everything," he added.

Sam Kiley, a security expert at UK-based Sky News, said France was heavily engaged in the fight against terror, which has made the country a target for terror attacks.

In Beijing, Cui Hongjian, director of the Department of European Studies at the China Institute of International Studies, said terrorists had targeted France as an influential country and wanted to "multiply the panic" with their attacks.

France has the largest number of Muslim migrants of any European Union country, and it has faced problems of cultural integration, Cui said.

Two Chinese nationals were among the injured, according to reports.

Tuo Yannan in Paris and Wang Qingyun in Beijing contributed to this story.

Contact the writers at chris@mail.chinadailyuk.com