NAIROBI, Kenya - The Somali militant group al-Shabab has formally joined al-Qaida, according to a video translation released of a message from al-Qaida's leader.

Ayman al-Zawahri gave "glad tidings" that al-Shabab had joined al-Qaida, according to the translation of the 15-minute video, which was released Thursday, by the Site Intelligence group.

"Today, I have glad tidings for the Muslim Ummah that will please the believers and disturb the disbelievers, which is the joining of the Shabaab al-Mujahideen Movement in Somalia to Qaedat al-Jihad, to support the jihadi unity against the Zio-Crusader campaign and their assistants amongst the treacherous agent rulers," he said.

Al-Shabab leaders have pledged allegiance to al-Qaida in the past, releasing a video in 2009 called "At Your Service Osama!" The same year, former al-Qaida leader Osama bin Laden released a video in which he made encouraging comments about the Somali insurgency.

But the new al-Zawahri video - which was posted on an Islamic Internet forum on Thursday - is the first formal welcoming of al-Shabab by the new al-Qaida leader. The new video also featured al-Shabab chairman Mukhtar Abu Zubeyr, also known as Godane, pledging allegiance to al-Zawahri.

Somalia's al-Shabab militia is a mix of conscripts, paid fighters, clan militias, and ideologues. It counts a few hundred foreign fighters among its ranks. Most are drawn from other East African nations but a few have travelled from as far afield as Pakistan and Chechnya.

The foreigners brought cash and tactical and bomb-making knowledge to al-Shabab, but the extent of al-Shabab's formal links to al-Qaida have often been unclear. Clan allegiances are still an important part of the 21-year-old Somali civil war, which currently pits al-Shabab against the weak U.N.-backed government. The government is supported by some 10,000 African Union troops and allied militias.

Last November al-Zawahri released a video about his memories of bin Laden. The longtime bin Laden deputy became al-Qaida's head after U.S. Navy SEALS killed bin Laden in May. Information in his compound at the time of his death suggested that bin Laden was "giving strategic direction" to al-Qaida affiliates in Somalia and Yemen.

Western intelligence officials say that al-Qaida officials have found sanctuary with al-Shabab for years. Last month, a U.S. drone strike killed Bilal al-Berjawi, a close associate of late al-Qaida operative Fazul Abdullah Mohammed, who directed the 1998 bombings of the U.S. embassies in Kenya and Tanzania that killed more than 200 people. Fazul was killed in Somalia last year.