* Uganda, Burundi make up the 8,000-strong force
* United Nations has given green light for 4,000 more troops
MOGADISHU, Jan 20 (Reuters) - The African Union (AU) has extended the mandate of its peacekeeping force in Somalia for another year, defying demands from rebels to withdraw, the AU's Peace and Security council said on Thursday.
The 8,000-strong force known as AMISOM, created in 2007 with an initial six-month mandate, is helping government troops counter an Islamist insurgency that has killed more than 21,000 people since early 2007.
Western spy agencies say Somalia has become a haven for militants and foreign jihadists bent on striking east Africa's main economies.
"(The AU) decides to renew the mandate of AMISOM for a supplementary period of 12 months, beginning from 17 January, 2011," the AU said in a brief written statement. Somalia's al Qaeda-linked al Shabaab group struck the Ugandan capital Kampala in July in a twin suicide bomb attack that killed 79 people. The rebels said the strike was to avenge the death of Somalis at the hands of Ugandan troops in Somalia.
The U.N. Security Council in December authorised the deployment of 4,000 additional troops to help shore up the western-backed interim government. Uganda is expected to provide the extra soldiers. (Editing by Richard Lough)
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