Somalia's hardline Shabaab Islamist group on Tuesday vowed to avenge the death of a top regional Al Qaeda leader during a lightning U.S. military operation on Somali territory.
According to U.S. officials and Western security sources, Saleh Ali Saleh Nabhan, a Kenyan citizen wanted by the FBI over the 2002 anti-Israeli attacks in Mombasa, was killed in a raid in southern Somalia on Monday.
The rare operation, which witnesses said involved several helicopters, dealt a blow to al-Qaeda's operations in East Africa but the Shabaab, an extremist jihadi group, pledged to strike back.
"Muslims will retaliate against this unprovoked attack," a top Shabaab leader told AFP. "The United States is Islam's known enemy and we will never expect mercy from them, nor should they expect mercy from us."
"We are investigating the matter and if any Somali is found to have aided the attackers, then he or she shall face Allah's verdict," he said on condition of anonymity.
The Shabaab official refused to elaborate on the circumstances of the operation, in which several other militants are believed to have been killed.
The area where Nabhan was killed, around 200 kilometers (120 miles) south of the capital Mogadishu, is firmly under the control of the Shabaab.
The Shabaab group has its roots in Somali Islamist movements created more than two decades ago.
The Shabaab, whose commanders were trained and fought in Afghanistan, spearheaded the bloody resistance against Ethiopia's two-year occupation of Somalia and are now involved in a deadly insurgency against the government.
Some of its leaders claim links to al-Qaeda and the movement is known to have sheltered known al-Qaeda operatives such as Fazul Abdullah, wanted over the Mombasa attacks as well as the deadly 1998 U.S. embassy bombings in Nairobi and Dar es Salaam.
Nabhan and Abdullah are suspected of involvement in recruiting some of the 500 foreign fighters currently believed to be in Somalia, fuelling fears that Somalia could become a new breeding ground for Osama bin Laden's organisation.
The U.S. has expressed concern that the Shabaab would turn Somalia into an extremist haven similar to the border area between Afghanistan and Pakistan -- which has been a top priority for the Barack Obama administration.
Nabhan and Abdullah are two of four top al-Qaeda militants behind the 2002 attacks in which an Israeli-owned hotel in Mombasa was bombed and rockets were fired at an Israeli airliner.
According to the U.S. network Fox, the U.S. operation was called "Celestial Balance" and involved helicopters backed by a navy ship.
According to Western security sources, six non-Somali militants -- including Nabhan -- boarded a vehicle on Monday morning in the coastal town of Merka, escorted by another car carrying three Shabaab fighters.
The vehicles were struck by one or several of the gunships shortly after the convoy stopped for breakfast, on their way to the southern Islamist stronghold of Kismayo.
The sources said bodies were taken by the helicopters for identification.