Al-Shabaab -- The Youth -- is losing ground to an offensive by Somalia's Western-backed Transitional Federal Government, backed by African peacekeepers, in Mogadishu, war-battered capital of the Horn of Africa state.
But with two dozen, possibly more, Somali-Americans reported to have joined its ranks, the group is seen as a threat to the United States. At least three of them have carried out suicide bombings in Somalia, raising fears their comrades could carry out similar operations in the United States.
Al-Shabaab lost its key commander, al-Qaida veteran Fazul Abdullah Mohammed, June 7. He was killed when he stumbled into a TFG checkpoint in Mogadishu.
He was a key link to al-Qaida Central in Pakistan and also coordinated with al-Qaida in the Arabian Peninsula based in Yemen, across the Gulf of Aden from Somalia, and one of the jihadists' most aggressive groups.
It's too early to determine whether al-Shabaab's vow of allegiance to Zawahiri will result in greater support for the Somali fighters.
But the Egyptian, who co-founded al-Qaida with Osama bin Laden in 1988, and other jihadist leaders have frequently called on the Somalis to attack African states and the West.
Al-Zawahiri was named al-Qaida's leader Thursday to replace bin Laden, who was killed May 2 by U.S. forces in Pakistan. Zawahiri had been his deputy and key strategist since the network was formed.
The SITE Intelligence Group, which monitors Islamist Web sites, said the pledge was made in a statement posted on the al-Andalus Islamic Station by the group's spokesman, Ali Mohammed Raghe, aka Ali Dheere.
"The mujahedin in Somalia confirm to the mujahedin in al-Qaida: You are more experienced than us and you have better views than us on the future of the jihad … against the enemy, as we have experienced from you before," the statement said.
"We await your instructions and we will act according to what you see in the coming stage to be in the interests of jihad the Muslim Ummah (nation)," it added.
"The mujahedin in Somalia say to the new emir: 'Here we are, here we are and we are waiting for your instructions."
In December 2010, Raghe publicly called on al-Qaida to send more fighters to Somalia "to expand the East Africa jihad."
Al-Shabaab has sought affiliation with al-Qaida since 2007 and pledged allegiance to bin Laden in 2008 but hasn't been formally merged with the jihadist network.
The presence of American citizens of Somali descent in al-Shabaab, part of a large contingent on foreign fighters with the group, is causing some anxiety in the U.S. security establishment.
Al-Shabaab's 3,000 hardcore fighters include several hundred foreigners, some of them al-Qaida veterans, U.S. and Somali intelligence estimates say. Most are Africans, but there are also Afghans, Pakistanis and Arabs.
Terrance Ford, director of intelligence with the U.S. Africa Command, observed in October 2010 that Americans occupy senior leadership positions in al-Shabaab and that "must be a strategic concern for the United States."
He said 43 of the group's 85-member executive council are foreigners, including Americans.
Omar Hammami, an Alabama native, is a top field commander with al-Shabaab. His nom de guerre is Abu Mansoor al-Ameriki and he has featured in propaganda videos to attract foreign recruits.
"The foreign jihadists were once in the shadows but now there's no doubt that they've taken control of the movement in Somalia," Ford noted.
This includes fighters from the United States. "All of us have concern about which way the foreign fighters flow, i.e., will they flow back into the United States?"
In November 2010, undercover FBI agents in a sting operation prevented a Somali-born American, 19-year-old Mohammed Osman Mohamud, from blowing up a van full of explosives at a crowded Christmas event in Portland, Ore.
In Britain, the director of the nation's domestic security service MI5, Jonathan Evans, warned in September 2010 that the country faced a growing threat from British residents trained by al-Shabaab in Somalia.
Evans said "a significant number of U.K. residents" were undergoing military training in al-Shabaab camps.
"It's only a matter of time before we see terrorism on our streets inspired by those who are today fighting alongside al-Shabaab," he declared.