Despite setbacks on the battlefield in Syria and Iraq the "Islamic State of Iraq and Syria" (ISIS) retains a "solid base of dedicated supporters" online, Europe's policing agency warned Friday (September 15th).

Europol warned that extremist groups are also now using smaller internet platforms and forums to spread terror propaganda, AFP reported.

"The relative decrease of 'official' ISIS propaganda over the last month has allowed more room for pro-ISIS user generated content," the agency said in a statement.

This indicates ISIS "continues to have a solid base of dedicated supporters in the virtual environment of the internet", it said.

The agency released its findings after a two-day joint operation earlier this week to seek out and flag content posted by extremist groups on the internet.

Europol's Internet Referral Unit teamed up with counter-terror and online propaganda experts from Bosnia and Herzegovina, the Czech Republic, Estonia and Hungary.

While extremist organisations and their supporters "continue to abuse a variety of platforms for disseminating their propaganda", Europol said, "the disruptive actions undertaken by certain service providers to safeguard their platforms" has driven sympathisers to "smaller" platforms.

Some sympathisers have reverted to using internet forums, as well as so-called "Darknet libraries" which share links pointing to extremist content on the open internet, Europol said.

The agency said it has flagged 1,029 pieces of content promoting terror on the internet to service providers with the request to have it removed if it violated their terms and conditions.