The hacker group, known for its cyber attacks against controversial organisations and individuals, is believed to have uploaded the footage on Friday night.
In the video, an announcer wearing an Anonymous Guy Fawkes mask and speaking French says: “You should know that we will find you and we will not let you go.
Announcer wears Guy Fawkes mask and delivers message in French (Anonymous)
“We will launch the biggest operation ever against you.
“Expect massive cyber attacks. War is declared. Get prepared.
“The French people are stronger than you and will come out of this atrocity even stronger.”
It is unclear why the video does not appear on the hacktivist group’sYouTube channel.
The hacker collective, which consists of unrelated volunteers, coders and activists from around the world, launched its anti-Islamic State online campaign, called #OpISIS, after the Charlie Hebdo massacre in Paris last January.
To date, according to an in-depth investigation by Foreign Policy, they have taken down 149 Islamic State-related websites and exposed 101,000 Twitter accounts and 5900 propaganda videos.
On Twitter alone, the seige is coordinated by four accounts, the latter three of which are bots – @CtrlSec, @CtrlSec0, @CtrlSec1and @CtrlSec2. Through these, they have unmasked the identities of thousands of ISIS-linked Twitter handles over the last nine months.
Although Anonymous was originally an apolitical hacking group, with no driving ideologies or agenda, it has recently become a defender of free speech on the internet.
Other social causes it has taken up include exposing members of the white nationalist Ku Klux Klan, supporting democracy advocates in Hong Kong and the Arab world, and organising hacking attacks against American agrochemical corporation Monsanto.
According to a March 2015 paper by the Brookings Institution titled “The ISIS Twitter Census,” there are anywhere from 46,000 to 70,000 Twitter accounts used by ISIS supporters between September to December 2014.
With ISIS, Twitter has been struggling to balance its support of free speech for all with clamping down on terrorist communications.
But according to the paper, Twitter had started to suspend large numbers of ISIS accounts by September 2014. About 1000 accounts were suspended between September and December 2014, and 10,000 accounts on April 2 of this year alone.
But this isn’t necessarily a good thing. “While suspensions appear to have created obstacles to supporters joining ISIS’s social network, they also isolate ISIS supporters online,” the authors write.
“This could increase the speed and intensity of radicalization for those who do manage to enter the network.”