The rebel group, which Washington says is al Qaeda's proxy in Somalia, hit the African Union's main base in Mogadishu with twin suicide car bombs on Thursday, killing 17 peacekeepers in a country of growing concern to Western security analysts.

The attack raised serious questions about the credibility of the nation's fragile UN-backed government, which controls just some of Somalia's central region and parts of the capital.

And in a sign of the insurgents' growing influence in the chaotic city, the rebels issued orders to schools on Saturday.

"I call upon all Somali parents not to send their youngsters to schools with curriculum supported by the UN agencies."

Fighting has killed more than 18,000 civilians since the start of 2007 and driven another 1.5 million from their homes.

Together with another rebel group, Hizbul Islam, al Shabaab has been battling government troops and the AU peacekeepers to impose its own strict version of Islamic law throughout Somalia.

Al Shabaab's stern religious views are rejected by many Somalis, who are traditionally moderate Muslims. But some residents do credit the gunmen with restoring relative stability and a measure of law and order to areas under their control.

Source: Reuters