Al-Qaeda's main Internet sites have gone silent for more than a week in an unprecedented blackout that is most likely the result of a cyber attack, analysts said Tuesday.

"All of them essentially went down" as of March 23, said Aaron Zelin, a researcher at the politics department at Brandeis University.

The outage hit several online forums including two "flagship" sites, al-Fida and Shamukh al-Islam, which serve as a channel for Al-Qaeda forums, providing a stamp of approval for any associated sites, Zelin said.

"The forums authenticate Al-Qaeda's message, therefore they're very important," said Zelin, who writes about extremist Islamist sites on jihadology.net.

"If someone is a true believer in the cause, they're going to go to the forum because they know it's the only place they can get Al-Qaeda's message."

One of the two main sites, Shamukh al-Islam, reappeared online on Monday but had not yet resumed message threads, he said.

A "second-tier" site, Ansar al-Mujahideen, was restored within three days, he said.

No one has claimed credit for the blackout, which bore all the signs of a cyber attack, analysts and former US officials said, as the forums usually post messages announcing a temporary interruption if they close the sites themselves.

The digital sabotage could have been carried out by any number of governments or private hackers, said James Lewis, director of the technology and public policy program at the Center for Strategic and International Studies.

"There are some many potential suspects," Lewis told AFP.

The long silence at the sites suggested Al-Qaeda was having difficulty getting service restored and was no longer as capable in the cyber realm.

"It's not a good sign for them that they can't straighten this out more quickly," Lewis said. "It could be seen as sign of decline."

US officials have already concluded Al-Qaeda has been steadily weakened since the attacks of September 11, 2001, and traffic on the group's online forums has declined in the past few years.

The number of Internet forums has declined from up to 13 in 2008 to no more than seven before last week's blackout, Zelin said.

The last time Al-Qaeda's sites faced a major outage was in June 2010, when British intelligence sought to disrupt the release of an online magazine from Al-Qaeda's branch in Yemen, according to Zelin.

Western officials have often debated the best approach to Al-Qaeda's online presence.

Although the group's forums are used to encourage terror attacks and spread extremism, the sites also provide a way for intelligence services to monitor the network's militants.

"There is always a debate over whether it's better to exploit or to disrupt. In this case, since one site came back up, it might be a way to funnel jihadis and make them easier to observe," Lewis said.

The blackout will help sow confusion among forum users, who will be doubly anxious about online surveillance and possible digital moles.

It was unclear if the blackout was linked to Mudhar Hussein Almalki, a Saudi national who was detained last week by Spanish authorities over his work with jihadist forums.

Almaki oversaw the Ansar Al-Mujahedeen network, which Spanish authorities described as online forums aimed at "recruitment, indoctrination and radicalization of sympathizers for armed jihad."