Internet investigators have described how they are locked in a "cat and mouse game" with Islamic State as they attempt to dismantle the terrorist group's vast and fluid online propaganda machine.

Officers face a "huge challenge" as thousands of supporters and operatives shift from one platform to another, according to a senior expert on a specialist European unit established to target terrorist material on social media and other sites.

The Press Association has learned the EU Internet Referral Unit (EU IRU) has identified 500 pieces of content, mostly relating to IS, in the two months since it was set up by Europol in July.

Referrals are passed to internet firms who then decide whether the material should be removed. In more than 90% of the cases, the content - such as videos of executions - has been deleted.

Officials estimate that there are more than 50,000 Twitter accounts used by supporters of IS, which is also known as Isil or Isis. As many as 100,000 tweets or other social media responses are being produced every day.

The expert, who cannot be named, said the battle against terrorism must be fought online as well as "offline".

"This is rather new for law enforcement," he said.

"That is of course our main challenge, trying to prevent them carrying out their message, threatening, planning."

He said that the propaganda network created by IS is "in a way sophisticated" and appears to respond to tactics adopted by law enforcement agencies.

"It is a huge challenge to give an answer to their daily innovations and improvements," he said. "It is, in a way, a cat and mouse game."

The expert cited a video showing an execution as an example of how quickly extremist content can spread.

"We saw that within the first 24 hours, 57,000 views, within the first 24 hours 32,000 tweets and retweets," he said. "150,000 views within the first 48 hours."

The unit's agents use tracking tools to identify content which appears on multiple platforms.

The expert said: "It's also about agility. They jump from one platform to another. It's not of any use to get one video out of the internet and then on the other platforms the video can still be watched."

IS's exploitation of technology to broadcast its barbaric acts and recruit new fighters is seen as unprecedented among terrorist groups and there are mounting fears that youngsters are at risk of radicalisation online.

It has been claimed that Talha Asmal, who reportedly became Britain's youngest suicide bomber while fighting for the group in Iraq, was "groomed" over the internet.

Europol director Rob Wainwright said IS's online activities reflect a deliberate attempt to "manipulate a new-ish part of our lives".

He referred to the "apparent indications of success" that IS is having in "turning the heads of impressionable 15-year-olds around Europe to become Jihadi brides or foreign fighters in Syria".

The unit is currently working with around 15 internet providers.

Mr Wainwright said they had been "very pleased" with the response of social media companies to the unit's activities amid questions over whether they do enough to help authorities.

He said: "There is a separate debate to be had about legislation around data communications, but what this also shows is there is a lot that can still be done through voluntary cooperation. "

A similar operation to the EU IRU was set up in the UK in 2010. It is currently removing material at a rate of around 1,000 pieces a week.