WASHINGTON—A key jihadist Internet forum has been knocked off the Internet, and counterterrorism experts say it appears it was hacked.

Cybersecurity analysts say the al-Shamukh forum appears to have been taken down by a fairly sophisticated cyberattack that hit not only the website, but the server—which is the main computer that enables people to access the site over the Internet.

Evan Kohlmann, a counterterrorism expert who tracks jihadist websites for Flashpoint Partners consultancy in New York, described the site as a key al-Qaida propaganda forum.

He said it bounces around between Internet hosts every few months but has seemingly been allowed to exist as an open secret, possibly allowing a Western government to use it as an intelligence resource.

NBC News first reported the site was hacked.