Dozens of westerners, including Britons, are known to have joined the ranks of Somalia's al-Shabaab, the Islamist militant group that has claimed responsibility for Saturday's deadly assault on the upmarket Westgate mall in Nairobi.

The terrorist group that pledged its obedience to al-Qaida and its head, Ayman al-Zawahiri, in February 2012, is fighting to impose a strict interpretation of Sharia law in Somalia.

Formed in 2006 as a spin-off of the now-defunct Union of Islamic Courts, al-Shabaab - which means 'the youth' in Arabic - has since succeeded in taking control of large areas of the lawless country.

The group turned Somalia into a safe haven for jihadi training, and foreign extremists poured into the country.

In 2010 former MI5 head Sir Jonathan Evans warned that as many as 100 Britons with Pakistani, Bangladeshi and West African backgrounds had flown to Somalia to receive terrorist training.

One year later, a similar report by the US House of Representatives Homeland Security Committee said that al-Shabaab had recruited more than 40 Muslim Americans and some 20 Canadians.

A member of the House Homeland Security Committee says the al-Qaida affiliated group that claimed responsibility for the deadly attack on a shopping mall in Kenya recruited up to 50 people from Somali-American communities in the U.S.

Committee member Rep. Peter King said that between 15 and 20 of Somali-Americans al-Shabab recruited remain active today. Speaking to ABC's This Week, the New York Republican also warned some may return and "use their abilities on the US."

Among the most famous examples are British native Samantha Lewthwaite, 29, the widow of one of the 7/7 London bombers, and Omar Hammami, 28, an Alabama native who came to international prominence when he posted a series of YouTube videos in which he rapped about jihad.

Omar Hamami al-shabaab
American-born Islamist militant fighter Omar Hamami, known as Abu Mansur Al-Amriki (Reuters)

Lewthwaite, nicknamed 'The White Widow' is the world's most wanted woman. She was allegedly part of a terror unit that planned to stage a bomb attack in the city of Mombasa, Kenya in 2011.

The widow of the Kings Cross bomber Germaine Lindsay, Lewthwaite earlier this year also allegedly plotted an armed assault on a Kenyan courthouse in order to free one of her British accomplices, Jermaine Grant.

Another British man said to be a close ally of Lewthwaite was reportedly killed in a gun battle in Somalia earlier this month.

Habib Ghani, 28, a bomb-maker of mixed British-Pakistani origin also known as Osama al-Britani, was reportedly executed by al-Shabaab after falling out with the group leadership.

He had joined a separate terrorist cell led by Hammami, known as Abu Mansur al-Amriki or 'the American', who was also killed.

Hammami had parted from al-Shabaab and founded his own militia last year, after terrorist leaders accused him of being selfish and loving the limelight.

The recruitment of westerners has reportedly slowed almost to a halt recently, thanks to a largely successful military offensive against al-Shabaab led by the African Union.

Al-Shabaab has been ousted from most town and cities and confined to rural areas.

The group said it launched the attack in Nairobi in retaliation against Kenya's involvement in these actions.

It is not known if foreigners are among the 20 extremists who broke into the four-storey mall and are still holding hostages nearly 24 hours after they attacked with grenades and assault rifles.

At least 59 people have been killed and 175 injured.

Samantha Lewthwaite
British-born terror suspect Samantha Lewthwaite