The fight against Islamic State jihadists is taking place online as well on the battlefield, with 18,000 Twitter accounts linked to the group suspended in recent months, according to a US expert.

IS 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.

Twitter has suspended nearly 800 confirmed IS accounts since the fall of last year but this "may be the tip of the iceberg," as almost 18,000 accounts "related" to the jihadist 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 IS group online have called the suspensions "devastating," Berger told the House Committee on Foreign Affairs.

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

But -- after IS 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 IS group's propaganda efforts, but has left intact less active accounts -- enabling spy agencies to monitor the group's backers, according to Berger.

"I believe the current environment is approaching the right balance of pressure on ISIS networks, degrading its ability to achieve its goals while still allowing the United States to exploit open source intelligence from the network of members and supporters online," he said.

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

The IS group has shown a particular acumen when it comes to exploiting social media, Berger said, and US commanders leading an international air war against the jihadists in Iraq and Syria have voiced concern over the effect of their propaganda.

According to Berger, the most active Twitter accounts "act in a coordinated way to amplify ISIS's 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 IS 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.