ISIS supporters "are under significant pressure, with the most active and viral users taking the brunt of the suspensions" J. M. Berger, a fellow at the Brookings Institution who tracks militants on social media, told lawmakers on Tuesday, AFP reports.

Twitter has suspended nearly 800 confirmed ISIS accounts since the fall of last year but this "may be the tip of the iceberg," as almost 18,000 accounts "related" to the militant network were suspended over the same time period, according to a forthcoming survey by Berger and another expert, Jonathon Morgan.

Although tens of thousands of Twitter accounts remain online, advocates for the ISIS group online have called the suspensions "devastating," Berger told the House Committee on Foreign Affairs.

The ISIS group had been able to operate on social media with relative ease until recently.

But - after ISIS released a grisly video in August of the beheading of American journalist James Foley - Twitter, Facebook and YouTube have instituted stricter rules to ferret out posts that support "terrorism," he said.

The suspensions have targeted the most active Twitter accounts, hampering the ISIS group's propaganda efforts, but has left intact less active accounts - enabling spy agencies to monitor the group's backers, according to Berger.

There are at least 45,000 Twitter accounts used by ISIS supporters, including those created and suspended in recent months, he said.

According to Berger, the most active Twitter accounts "act in a coordinated way to amplify ISIS' message, tweeting links to ISIS propaganda and hashtags at an unnaturally fast pace, which cause them to place higher in search results and results in content being aggregated by third parties."

The ISIS extremists also use numerous "bots," computer-directed Twitter accounts that automatically send out content to spread their message he said.

The study Berger cited was commissioned by Google Ideas and is due to be published in March.