Islamic State behind Paris 'act of war': Francois Hollande

French president Francois Hollande has denounced the terror attacks which killed at least 127 people in Paris as an "act of war" and blamed the Islamic State terror group 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.