UPDATE 3-Somali govt says rebels have more car bombs ready

* 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.