 -- ISIS has been under attack in the last few days from , which has quietly suspended at least 2,000 accounts linked to the terror group and its supporters, according to people with knowledge of the operation.

The sites shut down include some of the most important distributors of ISIS messages in a major escalation against ISIS’s propaganda and recruitment efforts, according to J.M. Berger, a terrorism analyst who monitors ISIS online messaging.

"Twitter has been doing a whole lot over the past week. They've slammed them pretty hard, including the official media distribution guys," Berger said. He said 13 of the 16 major ISIS distribution accounts were among those shut down.

Top U.S. security officials previously said ISIS has successfully leveraged social media networks, including Twitter, as powerful recruitment tools to draw in fighters from 90 countries. But a person with knowledge of Twitter’s recent suspension spree said it was not done due to U.S. government pressure -– in fact, he said the U.S. intelligence community would prefer the accounts stay open for intelligence gathering purposes.

Instead, the suspensions have been a result of increased media reporting, which in turn spurred public awareness, which has created more user-generated policy violation reports, the person said. It’s against Twitter’s policy, for instance, to make direct, specific threats of violence against others.

The assault on ISIS social media was followed by an apparent threat Monday by supporters of the Syria-based network responsible for killing thousands of innocents while establishing an Islamic "caliphate" from Syria to Iraq and elsewhere. The message posted online called out Twitter founder Jack Dorsey by name and used his photo.

The Twitter spokesperson would only say that the company's security team was "investigating the veracity of these threats with relevant law enforcement officials," and that content is constantly reviewed against Twitter's rules.

"I would certainly be concerned if I were Twitter," said Berger, co-author with Jessica Stern of "ISIS: The State of Terror," set for release next week.

A counter-terrorism official told ABC News that the U.S. was looking into the threat and that for months ISIS's ability to distribute its daily dose of beheading and murder videos has been "challenged." Some airstrikes by the U.S.-led coalition in Iraq and Syria have targeted the ISIS media team, which analysts have assessed has limited the core leadership from releasing more than a handful of slick videos since September.