The group is said to control most of the southern and central parts of the country, including a large swath of the capital, Mogadishu, where it has imposed its own harsh form of Sharia law.
The group was formerly the military wing of the deposed Islamic Court Union (ICU) that controlled much of central and southern Somalia in late 2006.
The ICU splintered into several smaller groups after its removal from power by Ethiopian troops in support of the largely powerless UN-backed interim government.
Al-Shebab describes itself as waging jihad against the “enemies of Islam” and is engaged in combat against the Somali Transitional Federal Government (TFG) and African Union Mission to Somalia (Amisom).
It has declared war on the UN and on Western NGOs that distribute food aid in Somalia.
Al-Shebab, which murdered 42 relief workers between 2008 and 2009, is reported to be tied to al-Qaeda. Its leaders denied that such a link existed until early 2010.