FRENCH president Francois Hollande has denounced the terror attacks which killed at least 127 people in Paris as an "act of war", as the Islamic State terror group claimed responsibility for the carnage.

Speaking after an emergency meeting of senior government and security officials at the Elysee Palace, Mr Hollande announced three days of national mourning and vowed that France would be "pitiless" in its response to terrorism.

A manhunt is under way for accomplices of gunmen who targeted a concert hall and the French national football stadium and sprayed the terraces of bars and restaurants with gunfire in at least six separate attacks.

French authorities said they believed all eight of those involved in the attacks were dead - seven of them killed by suicide bombs - but Paris's chief prosecutor said it was possible other terrorists were still on the run.

Policing was being strengthened at ports and major events in the UK, and Prime Minister David Cameron was due to chair a meeting of the Government's Cobra emergency committee which could raise the official assessment of the threat from international terrorism from its current "severe" level.

In a televised address to the nation, Mr Hollande said the attacks were "committed by a terrorist army, the Islamic State group, a jihadist army, against France, against the values that we defend everywhere in the world, against what we are: a free country that means something to the whole planet".

He added that France "will be merciless toward the barbarians of Islamic State group", and "will act by all means anywhere, inside or outside the country".

Mr Hollande said the French army and security forces were mobilised "at the highest possible level" and insisted France would "triumph over barbarity".

"What we are defending is our country, but more than that, it is our values," he said.

The IS claim of responsibility for the attacks was made in a statement in Arabic and French released online and circulated by supporters of the group.

It was not immediately possible to confirm the authenticity of the statement, but it bore the group's logo and resembled previous statements.