* Minister says insurgents have 8-car plan
* Blasts raise fears about government credibility
* Burundi wants AU force to be more aggressive
(Adds weapons appeal, paragraphs 10-11)
By Abdi Sheikh
MOGADISHU, Sept 18 (Reuters) - Somalia's al Shabaab insurgents have six more stolen United Nations vehicles primed as suicide bombs, the government said on Friday.
President Sheikh Sharif Ahmed's administration says it will not be bowed by twin suicide car bombs that hit the African Union's (AU) main base in Mogadishu on Thursday, killing 17 AU peacekeepers including the AMISOM force's deputy commander.
But the audacious attack by two U.N.-marked cars on the heart of the peacekeeping mission raises serious questions about the credibility of the deeply divided government, which controls little more than a few districts of the capital.
The state minister for defence, Sheikh Yusuf Mohamed Siad, a former warlord also known as "Inda'ade" or "white eyes", said the insurgents had seized more U.N. vehicles in recent months.
"We were all aware of their suicidal preparations but we never thought they would penetrate the AMISOM compound," he said. "We knew they were masterminding eight cars ... they are left with six more cars. That is cowardice." Inda'ade said the bombings would not stop the government launching fresh attacks against al Shabaab, which Washington says is al Qaeda's proxy in the failed Horn of Africa state.
"People will see what we'll do to them. They are not Muslims ... We know each other. Let's wait and see what happens next."
Al Shabaab gunmen, including foreign fighters, have stolen vehicles in raids on U.N. compounds in Somalia in recent months.
Thursday's attack was the worst yet on the 5,000-strong AU force of soldiers from Uganda and Burundi. Its Ugandan commander was also wounded in the explosions.
Twelve of the dead AU peacekeepers were from Burundi's army, which urged the AU on Friday to toughen AMISOM's mandate to let it conduct offensive operations against the rebels.
"The current mandate hampers our job. We want the AU to change it and let our troops chase the insurgents to their last hideout," said its spokesman, Major General Lazard Nduwayo.
Nicolas Bwakira, the AU envoy to Somalia, called for donor countries to reinforce AMISOM with more troops and heavier guns.
"We don't need toys. We need arms that are superior to those of al Shabaab," he told reporters in neighbouring Kenya, where dozens of the most badly wounded were flown for treatment.
Thursday's blasts at the heavily guarded heart of the mission followed one of Mogadishu's most violent months in 20 years. Fighting in Somalia has killed more than 18,000 civilians since the start of 2007 and left 1.5 million more homeless.
Western security agencies say the lawless nation has become a safe haven for militants, including foreign jihadists, who are using it to plot attacks across the region and beyond.
Al Shabaab said the strike was in revenge for the killing in southern Somalia of one of Africa's most wanted al Qaeda suspects in a helicopter raid on Monday by U.S. special forces.
Kenyan-born Salah Ali Saleh Nabhan was wanted for the 2002 bombing of an Israeli-owned Kenyan hotel that killed 15 people.
But many Mogadishu residents denounced the rebels on Friday for retaliating with an attack that claimed only the lives of four Somali civilians and east African peacekeepers. At least 19 locals were also killed in shelling after the blasts.
"Bombing Somali Muslims because of a dead foreign terrorist is totally ungodly and inhumane," businesswoman Asha Farah told Reuters. "I can only say that al Shabaab are mad."
The U.S. raid that killed Nabhan likely won Washington valuable intelligence, but risked further inflaming anti-U.S. opinion in a country of growing concern to the West.
Peter Pham, a U.S.-based analyst, said the Somali government (TFG) was in severe difficulties. But he warned that it would be a mistake of "startling proportions" if Washington sent more weapons to help it battle the rebels.
"The TFG is both so corrupt and so lacking in capacity that sending it materiel has only made it more convenient for the insurgents fighting it -- who are well-financed thanks to their foreign donors, both state and non-state -- to simply replenish their arsenals on the open market," Pham wrote. (Additional reporting by Ibrahim Mohamed, Mohamed Ahmed and Abdi Guled in Mogadishu, Patrick Nduwimana in Bujumbura and Sahra Abdi and Frank Nyakairu in Nairobi; Writing by Daniel Wallis; Editing by Giles Elgood and Jon Hemming)