The hacker organization Anonymous on Monday declared itself at war with the Islamic State over attacks in Paris that killed at least 129 people.
In an unverified video posted on YouTube, an Anonymous spokesman, speaking French and wearing the group's signature Guy Fawkes mask, threatened IS -- also identified as Daesh, ISIS and ISIL.
"Expect massive cyberattacks. War is declared. Get prepared. Anonymous from all over the world will hunt you down. You should know that we will find you and we will not let you go. We will launch the biggest operation ever against you. The French people are stronger than you and will come out of this atrocity even stronger," the speaker in the video says.
The group's Twitter account confirmed Sunday it was at war with IS, which has claimed responsibility for a series of bombings and shootings Friday in Paris.
Anonymous has a history of launching cyberattacks against entities with which it disagrees. PayPal, Mastercard and the Church of Scientology are among the victims of its attacks.
Anonymous temporarily took down a French jihadist website in January, following the killing of 12 people at the Paris-based magazine Charlie Hebdo. The attackers identified themselves as members of al-Qaida's Yemen branch.
Earlier this month the hackers identified 1,100 alleged members of the Ku Klux Klan, posting the names on its website. It also organized a worldwide protest, known as the Million Mask March, earlier in November.