Al-Qaida's leader announced in a video Thursday that the Somali militant group al-Shabab has formally joined the Islamic extremist network, fusing two groups that have been targets of America's counterterrorism efforts.

Today, I have glad tidings for the Muslim ummah [world community] that will please the believers and disturb the disbelievers, which is the joining of the Shabaab al-Mujahideen Movement in Somalia to Qaidat al-Jihad to support the jihadi unity against the Zionist-Crusader campaign and their assistants amongst the treacherous agent rulers, al-Qaida leader Ayman al-Zawahiri said in the video.

It is unclear what difference making the arrangement official will make. The Obama administration has already launched drone strikes against al-Shabab, acting under the same post-Sept. 11, 2001 powers that allow the U.S. president to deploy force against al-Qaida. Informal linkages have long existed between the two groups, although al-Shabab's aim of imposing Sharia law in Somalia is narrower in scope than al-Qaida's vision of global jihad.

Al-Shabab has also become increasingly weakened as the Somali populace it depends on for tribute has turned against it. African Union soldiers have been successful in routing al-Shabab forces in Mogadishu, the Somali capital, while Kenyan troops have kept up the pressure since entering Somalia in October.

The clans and militias that opportunistically aligned with al-Shabab and provided the bulk of their forces' strength, many of them are peeling away, said J. Peter Pham, director of the Africa program at the Atlantic Council. In a way it doesn't add much to al-Qaida to pick up a group that's in serious decline and could be militarily defeated.

Still, the newly forged partnership could allow al-Qaida to recruit some of the more hardline members of al-Shabab for operations outside of Somalia, Pham said.

The danger there is the Shabab have shown they have links to the Somali diaspora to Europe, North America, Australia and other places, and those could be put in the service of al-Qaida, Pham said.