LONDON - A new ally in the war on terror has emerged in the wake of the massacre at the office of Paris' Charlie Hebdo magazine - on YouTube.

The group of "hacktivists" - activist computer hackers - that calls itself Anonymous posted a video in which a masked member, speaking in French, declares "war on you, the terrorists" for the attack in which 12 people were murdered, singling out the Islamic State in Iraq and Syria (ISIS) and Al Quaeda.

"We will track you down - every last one - and will kill you," the spokesman says, according to a translation of one of its videos in the Mirror online. "You allowed yourselves to kill innocent people, we will therefore avenge their deaths."

Wearing the Guy Fawkes mask popularised in the Natalie Portman movie V For Vendetta and speaking in an electronically disguised voice, the video was posted by the Anonymous group's Belgian branch and accompanied by a statement on Pastebin.

The speaker calls on "hacktivists" the world over to hunt down extremists online and join hands to shut down their Facebook, YouTube and Twitter accounts.

"You will not impose your sharia law in our democracies, we will not let your stupidity kill our liberties and our freedom of expression," says the masked person. "We have warned you; expect your destruction."

Anonymous is a group of hackers and activitists purporting to protect and support democracy. The masks they wear are of Guy Fawkes, a 17th century Roman Catholic activist who plotted to blow up England's Parliament and King James 1 for suppressing his religion.

Anonymous has reportedly been behind attacks of corporate, religious and government websites, where it blocks access by users by flooding them with traffic from all round the Internet, a method known as a distributed denial of service (DDoS) attack.

The group reportedly has coordinated such cyberattacks on China's government webistes in support of pro-democracy protesters in Hong Kong by attacking Chinese government websites.

In the accompanying Pastebin statement, which it calls "a message to the enemies of freedom of expression", Anonymous offers condolences to "the families of the victims of this cowardly and despicable act" in Paris.

"Freedom of expression and opinion is a non-negotiable thing to tackle - to attack it is to attack democracy," the statement concludes.

"Expect a massive and head-on reaction on our part because the defence of those freedoms is the foundation of our movement.

"We will track you everywhere on the planet, nowhere will you be safe. We are Anonymous. We are legion. We do not forget. We do not forgive. Be afraid of us, Islamic State and Al Qaeda - you will get our vengeance."

An English version of the video posted by the group: