* Say donor-funded curriculum is un-Islamic
* Move follows twin suicide car bomb attacks
* Regional body IGAD says Eritrea backs al Shabaab rebels
MOGADISHU, Sept 20 (Reuters) - Somalia's hardline al Shabaab insurgents have warned schools not to use textbooks provided by U.N. agencies and other donors they accuse of being un-Islamic.
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 U.N.-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.
"Some U.N. agencies like UNESCO are supplying Somali schools with text books to try to teach our children un-Islamic subjects," al Shabaab spokesman Sheikh Ali Mohamud Rage told Koranic students gathered at Mogadishu's Nasrudin mosque.
"I call upon all Somali parents not to send their youngsters to schools with curriculum supported by the U.N. 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.
In July, the group barred three U.N. agencies from operating on its territory, saying the U.N. Development Programme, U.N. Department of Safety and Security and U.N. Political Office for Somalia were working against the creation of an Islamic state.
REGIONAL BODY SLAMS ERITREA
Thursday's attack on the heavily guarded heart of the AMISOM peacekeeping mission, next to Mogadishu's main airport, was the worst yet on the force of 5,000 troops from Burundi and Uganda.
The deputy commander of the mission was killed and the commander wounded when two U.N.-marked cars drove into the base before exploding. Burundi was burying its 12 dead on Sunday.
The Somali government warned on Friday the insurgents had six more stolen U.N. vehicles primed as suicide car bombs.
The United Nations is investigating the use of its cars, which were thought to have been seized in rebel raids on U.N. compounds in central Somalia in May and July. [ID:nLJ475178]
Mark Bowden, the U.N. aid coordinator for Somalia, told Reuters the cars could have come from as far away as a former U.N. mission in Eritrea, but that there were many U.N. vehicles in Somalia that had been used for a wide range of projects.
Eritrea has repeatedly denied U.S. allegations that it is arming and funding al Shabaab. On Saturday, a senior official at the east African regional body IGAD said it had proof it was.
"We have conclusive evidence that Eritrea and al Qaeda are supporting, abetting and financing the terrorists," Kiprute Arap Kirwa, IGAD's peace and reconciliation facilitator for Somalia, told reporters in the Ethiopian capital Addis Ababa.
Kirwa did not elaborate, but called on the international community to take immediate and effective action.
Al Shabaab said the suicide attack was in revenge for last Monday's killing by U.S. special forces of one of Africa's most wanted al Qaeda suspects in remote rebel-held southern Somalia.
On Saturday, President Sheikh Sharif Ahmed said his administration had given Washington permission to hunt down Saleh Ali Saleh Nabhan -- a 28-year-old Kenyan wanted for the 2002 truck bombing of an Israeli-owned beach hotel in Kenya that killed 15 people -- because it could not catch him. (Additional reporting by Tsegaye Tadesse in Addis Ababa; writing by Daniel Wallis; editing by Robin Pomeroy)
Our Standards: The Thomson Reuters Trust Principles.