Its name stems from its first incarnation as the youth and military wing of a group of Sharia courts who controlled much of southern and central Somalia in 2006.

When, at the end of that year, the Union of Islamic Courts (UIC) was driven from power by Ethiopian troops supporting Somalia's weak transitional government, al-Shabab melted away into remote and distant parts of the country.

But it did not disappear.

Pattern

It re-emerged as a radicalised group of young fighters, who, for the past 18 months have been conducting an insurgency against the interim government and its Ethiopian allies and attacking African Union peacekeepers.

Al-Shabab feels honoured to be included on the list. We are good Muslims and the Americans are infidels   Al-Shabab's Muktar Robow

Unlike many UIC leaders, who fled into exile in Eritrea, al-Shabab stayed behind to fight.

It has its own website and has become increasingly active, not only fighting in the capital, Mogadishu, but carrying out hit-and-run attacks throughout central and southern Somalia.

In the past few weeks, al-Shabab has attacked a number of strategic towns, including Dinsor in the south-west and Bur Hakaba, near the seat of parliament in Baidoa.

A pattern is emerging whereby the militia briefly occupy the town, often killing a number of people, then withdraw with arms, ammunition and military vehicles seized from Somali government and Ethiopian troops.

Last week, a Somali soldier was beheaded after the group issued a warning that it would attack checkpoints and behead those operating them.

Al-Qaeda

Al-Shabab has distanced itself from the Somali opposition based in Eritrea, saying it is too secular.

Checkpoint soldiers have been threatened with beheading

What started as a purely Somali armed group has become a more serious threat.

"Al-Shabab is a violent and brutal extremist group with a number of officials affiliated to al-Qaeda," the US State Department said in a statement.

Led by Aden Hashi Ayro, who is said to have trained with al-Qaeda in Afghanistan, al-Shabab is reported to have attracted members from Yemen, Pakistan and various African countries.

It is this that worries the Americans.

But ironically the fact that the US has repeatedly made statements linking al-Shabab with al-Qaeda may have made it more attractive to some of the foreign fighters.

A senior member of the al-Shabab, Sheikh Muktar Robow, told the BBC he welcomed the US decision.

"Al-Shabab feels honoured to be included on the list. We are good Muslims and the Americans are infidels. We are on the right path," he said.

But he rejected the US's accusations that members of the group are linked to al-Qaeda.

"We are fighting a jihad to rid Somalia of the Ethiopians and its allies, the secular Somali stooges," he said.

After repeating that al-Shabab would not stop fighting or engage in reconciliation talks until Ethiopian troops withdrew from Somalia, he said: "We will continue to attack peacekeepers in Somalia, regardless of their nationality."