Sombre Hollande denounces third 'terrorist' act in 18 months

There was a sense of deja vu in France as the visibly moved president took to the airwaves to address a nation once again in mourning.

If confirmed as an act of terror, the incident will be the third major attack on French soil in 18 months -- with several smaller-scale militant killings also having taken place.

While no group has claimed responsibility for the attack in the resort city, Hollande vowed to strengthen his country's role in the fight against the Islamic State (Daish) group in Iraq and Syria.

"Nothing will make us yield in our will to fight terrorism. We will further strengthen our actions in Iraq and in Syria. We will continue striking those who attack us on our own soil," he said, in reference to the IS group.

Hollande said several children were among the dead after the attack, which he said was of an "undeniable terrorist nature".

He vowed ever stronger security measures -- calling up reservists and extending a state of emergency -- as he reached for familiar, and new words to boost the morale of a battered nation.

"France is horrified by what has happened, this monstrosity which is using a truck to deliberately kill dozens of people who simply came to celebrate July 14.

"France was struck on its national day, a symbol of freedom," said Hollande.

France "will always be stronger, I promise you, than the fanatics that want to strike it."

The Islamic State group has repeatedly singled out France as a prime target for its military actions against the group in Iraq and Syria, and hundreds of extremists have left France to go and fight in its ranks.

The country has been under a state of emergency ever since militants killed 130 in Paris on November 13, and the government has boosted its security laws.

Just hours before the attack, Hollande said the state of emergency would not be renewed beyond July 26 after the adoption of a new law in May bolstering security.

However, after the incident, he said it would be extended for another three months.

While security forces will remain on high alert, Hollande also called on France's "operational reservists" to boost the ranks of police and gendarmes.

These include French citizens with or without military experience as well as former soldiers.