UPDATE: This article has been updated to include the reactions of some of the companies involved.

Online hacktivist collective Anonymous has released a comprehensive list of websites suspected to be linked to Islamic State (Isis), together with the companies responsible for hosting the content. Many of the web hosting companies are UK, Europe and US-based, provoking criticism that they are complicit in disseminating Isis propaganda used to recruit members and incite terror attacks.

The latest release forms part of Op Isis (#OpISIS), an ongoing cyber campaign carried out by the amorphous group designed to disrupt Isis' online operations and "cure the Isis Virus". It was compiled by GhostSec, a prominent faction of Anonymous that has been monitoring Isis through its social media channels.

"These websites were collected by monitoring Islamic State accounts on Twitter and logging them over a period of three months," an Anonymous member involved in compiling the list told IBTimes UK.

"We believe it is critical for the world to see who is hosting this type of content and we believe they should be held accountable. These web assets pose a great risk as they are a source of recruitment for Islamic State and a communications platform used to conduct atrocities against humankind."

The list includes prominent companies like CloudFlare and Yahoo! Europe, responsible for providing server space for the domain name and protecting the sites from cyber threats like DDoS (distributed denial of service) attacks.

All websites listed below are frequently used by the Islamic State through Twitter and other social media platforms for transmission of propaganda, religion, recruitment, communications and intelligence gathering purposes. (Web host companies) are unaware of their customers content or they turn a blind eye for easy profit and choose to accept bloodmoney. CloudFlare is by far the largest offender on this list and they have been made aware of the specified content they are protecting... Together we can stop this from spreading and hold these companies accountable for their less than ethical business practices.

GhostSec statement

anonymous ghostsec opisis isis
A Twitter profile linked to one of the listed websites offers proof of extremist links Digital Shadow

'Extremely difficult' to police

Many of the websites listed do not appear to have any affiliation with jihadist groups, however links can be found to extremists and supporters of Isis when traced back to related social media accounts.

As an example, the site dr-algzouli.com, hosted by Web Hosting UK (WHUK), can be linked to a Twitter profile of the same name that expresses support for a caliphate under Islamic State rule.

WHUK said that the large number of websites that it hosts means that it is hard to monitor what content is appearing on them.

"We take matters like this very seriously and actively remove websites such as this, that breach our acceptable use policy as soon as we are made aware of them," Frank Tighe, chief technology officer of Hyperslice, told IBTimes UK when the matter was brought to his attention. "The offending website has now been removed.

"Whilst we try to ensure that all our customers adhere to our acceptable use policy at all times, we host hundreds of thousands of websites and logistically speaking it's extremely difficult for us to keep track of the content of all of them all of the time."

Hundreds of websites allegedly associated with Isis have already been attacked by Anonymous and its affiliates. Most recently Anonymous claims to have attacked 233 websites and "destroyed" 85 of them as part of OpIsis.

Lists of Twitter accounts related to Isis have also been released, however the methods employed by Isis members and its supporters have meant that having them reported and removed has so far proved ineffective.

A separate source within Anonymous previously told this publication that Twitter provides a Hydra-like platform for Isis to thrive.

More than 25,000 Isis-related Twitter accounts have been released, however thousands more have since taken their place.

This article will be updated with comments from CloudFlare and other companies if they respond to a request for comment.