MOGADISHU: Somalia's hardline Shebab Islamist group on Tuesday vowed to avenge the killing of a top regional Al-Qaeda leader during a lightning US military operation on Somali territory.

Saleh Ali Saleh Nabhan, a Kenyan citizen wanted by the FBI over the 2002 anti-Israeli attacks in Mombasa, was killed in an airborne raid in southern Somalia on Monday, according to US officials and Western security sources.

The regional governor said on Tuesday that three Somali civilians had also been killed in the US attack.

The rare operation, which witnesses said involved several helicopters, dealt a blow to Al-Qaeda's operations in East Africa but the Shebab, an extremist jihadi group, pledged to strike back.

"Muslims will retaliate against this unprovoked attack," a top Shebab 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 Shebab official refused to elaborate on the circumstances of the operation, in which several other militants are believed to have been killed.

Meanwhile, the governor of the Lower Shabelle region where the attack happened told a press conference in Mogadishu that three civilians were also killed in the operation, the details of which remain sketchy.

"There were casualties in the attack carried out by the Americans in the region: two civilians in a nearby vehicle and a woman on the scene of the attack. Five other civilians were also wounded," Abdulkadir Sheikh Mohamed said.

"They carried out this attack without telling us anything. It is not fair because there were civilian casualties," he added.

The governor's statement was not immediately confirmed by local medical sources or Shebab officials.

The area where Nabhan was killed, around 200 kilometres south of the capital Mogadishu, is firmly under the control of the Shebab.

The Shebab - 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 Al-Qaeda operatives such as Fazul Abdullah, wanted over the Mombasa attacks as well as the deadly 1998 US 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 US has expressed concern that the Shebab would turn Somalia into an extremist haven similar to the border area between Afghanistan and Pakistan - the eradication of which has been a top priority for the 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.

Israel and the United States, whose top diplomats recently visited the region, have in recent years complained of the slow progress in efforts to hunt down key suspects in the 1998 and 2002 attacks.

According to the US network Fox News, the US 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 Shebab 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 to one of the US fleet's nearby vessels for DNA tests to be carried out.

The US military launched a missile strike targeting Nabhan in March 2008 near the border with Kenya but Monday's successful ground and air operation was a first against Al-Qaeda in Somalia. - AFP/de