WASHINGTON (AFP) - US authorities and Internet giants are boosting attempts to counter the Islamic State in Iraq and Syria's online propaganda, though it is unclear how effective these efforts are in hampering the extremists' public-relations machine.

With calls to jihad and highly produced videos of ISIS fighters in battle or killing captives, ISIS has long used the Internet and social media to recruit fighters for its so-called caliphate in Iraq and Syria, and to incite individuals around the world to commit terrorist attacks.

To try to stop this, web giants like Twitter and Facebook are working hard to shut down extremist accounts, though these often pop back up under a new name.

"Twitter has publicly said they've taken down close to 200,000 handles. They've taken down way more than that," Richard Stengel, the under secretary of state for public diplomacy and public affairs said at a recent seminar aimed at countering ISIS's "brand." "YouTube has taken down literally millions of videos. Facebook has hundreds of people who are working 24/7 to take down this noxious content," added Stengel, who also was the former managing editor of Time magazine.

At the same time, US efforts are being led by the State Department's Global Engagement Centre, which was overhauled this year and brought under the leadership of Michael Lumpkin, a former US naval officer.

The centre "is not going to be focused on US messages with a government stamp on them, but rather amplifying moderate credible voices in the region and throughout civil society," said Lisa Monaco, President Barack Obama's top homeland security advisor.

The US military's Central Command, which oversees operations in the Middle East, is "actively engaged" on social media to counter ISIS propaganda.

"The command has a robust online engagement programme that harnesses the professional talents and expertise of both military members and contractors working together," Pentagon spokesman Major Adrian Rankine-Galloway said.

"We operate using truthful information directed towards regional audiences to combat ISIL's lies and deception," he added, using an ISIS acronym.

Stenger said efforts are paying off.

"There's now five times as much messaging on social media that is anti-ISIL than pro-ISIL, again, mostly in Arabic," he said.

JM Berger, an expert on the ISIS group who has written extensively about the extremists' use of Twitter, said they are feeling the squeeze.

"There is no question that ISIS supporters on Twitter and elsewhere are under tremendous pressure, and they are performing significantly below the levels we saw last year, or even earlier this year," he said.

"Supporter accounts have fewer followers and tweet less often. They are still able to distribute their propaganda to a shrinking core audience, but it is harder for them to broadcast widely and to get their message in front of potential recruits."

Will McCants, an expert on extremists at the Brookings Institution think tank, said pressure on Facebook and Twitter has seen ISIS supporters turn to smaller social media platforms like Telegram to disseminate their propaganda.

"Still, they try to maintain a presence on the larger platforms because that's where the potential recruits are," he noted.

Rita Katz, who co-founded the private intelligence firm Search for International Terrorist Entities Intelligence Group (SITE), rejected the notion that extremist propaganda was slowing.

ISIS propaganda "at least doubled last year if not even more. Further, in addition to the daily reports, in the last year the Islamic State has also increased substantially its publications, as it started several new ones," she said, pointing to new Turkish- and Russian-language magazines.

"These groups and individuals are still online and they're still recruiting," she said.

Meanwhile FBI Director James Comey noted that while there has been a drop in people traveling to join ISIS, the group retain the ability to "motivate troubled souls."