(CNN) -- Somalia's Al-Shabaab rebel movement has tightened its ties to the al Qaeda terror network, with its leader pledging loyalty to al Qaeda leader Ayman al-Zawahiri.
The Islamist militants control much of southern Somalia and have long been affiliated with al Qaeda. But in an audio message posted on Al-Shabaab's website Thursday, the Somali group's leader, Mukhtar Abu al-Zubair, tells al Qaeda leader Ayman al-Zawahiri that his followers "will march with you as loyal soldiers."
Al-Zawahiri took command of al Qaeda after U.S. commandos killed the movement's founder, Osama bin Laden, last May. Al-Zubair addresses al-Zawahiri as "my dear commander and kind sheikh" and congratulates him "for the defeat of the crusaders in Afghanistan and Iraq."
"On behalf of the soldiers and the commanders in al-Shabaab, we pledge allegiance to you. So lead us to the path of jihad and martyrdom that was drawn by our imam, the martyr Osama."
And in a video message posted on the site, al-Zawahiri hails the move as news "that will delight the believers and annoy the crusaders."
"The Arab region is slipping from America's grip," he says.
The combined messages run nearly 15 minutes.
Al-Shabaab is already considered a terrorist movement by the United States. It has been battling Somalia's U.N.-backed transitional government and African Union peacekeepers for the past several years, but has suffered a series of setbacks in recent months.
AU and government forces drove Al-Shabaab fighters from the center of the Somali capital Mogadishu last year, while Kenyan troops crossed into southern Somalia in October to hit back for a rash of kidnappings it blamed on the group.
In November, the leaders of Kenya, Somalia and Uganda reaffirmed their commitments to battle Al-Shabaab, and U.S. officials say the group has recruited Somali-Americans to take up arms in their ancestral home.