Nairobi - One of several suicide bombers who killed 21 people, including 17 African Union peacekeepers, at a base in the Somali capital Mogadishu September 17 was American, a Somali-language website has claimed.

Militants from Islamist insurgent group al-Shabaab - which the US says has close links with al-Qaeda - entered the base in vehicles stolen from the United Nations and detonated explosive charges as a meeting between Somali officials and peacekeepers was taking place.

Somali-language website Dayniile. com, without revealing its sources, reported that one of the bombers was Omar Mohamed Mahmoud, a Somali-American who lived in United States until 2007.

The site is run by members of the Mursade - a subclan of the large Hawiye clan - which has provided a significant number of fighters to al-Shabaab.

Gaffel Nkolokosa, a Nairobi-based spokesman for the AU peacekeeping mission in Somalia (AMISOM), told the German Press Agency dpa that investigations, including DNA analysis of remains, were ongoing and that the identity of the bombers had not yet been confirmed.

The US Embassy in Nairobi declined to comment. No one from al- Shabaab was immediately contactable.

Should the report prove to be true, it will be the second verified case of an American citizen turning suicide bomber in Somalia.

Shirwa Ahmed became the first known naturalized US citizen to become a suicide bomber when he blew himself up in the self-declared autonomous Somali region of Puntland last October, killing dozens.

FBI director Robert Mueller said that Ahmed was radicalized in the US state of Minnesota, which has a sizable Somali community.

The FBI believes that over a dozen Somali-American youths have left Minneapolis to join al-Shabaab over the last two years.

Jamal Bana, 20, and Burhan Hassan, 17, both former residents of Minneapolis, were shot dead while fighting for al-Shabaab.

The bombing of the base came days after al-Shabaab vowed to retaliate for a US raid, which killed al-Qaeda suspect Saleh Ali Saleh Nabhan and several al-Shabaab fighters.

Diplomats in Nairobi say al-Qaeda is building its capabilities in Somalia and is looking to launch terror attacks across the region.

Kenyan police recently claimed to have foiled a plot to bomb the hotel where US Secretary of State Hillary Rodham Clinton was staying during a visit to Nairobi in early August.

Together with its ally Hizbul Islam, al-Shabaab has been battling to remove Western-backed President Sheikh Sharif Sheikh Ahmed.

Somalia has been embroiled in chaos since the 1991 ouster of dictator Mohamed Siad Barre.

The current insurgency kicked off in early 2007, following an Ethiopian invasion, and has recently gathered pace.

More than 250,000 people have fled renewed fighting in Mogadishu since May, bringing the total number of displaced within Somalia to over 1.5 million. Over 18,000 civilians have died since early 2007. (dpa)