(Adds government comment, paragraph 10)

By Abdi Sheikh

MOGADISHU, Oct 11 (Reuters) - Two African Union military planes braved rebel threats to land in Mogadishu on Saturday carrying 400 Burundian reinforcements for a peacekeeping force.

The AU mission is guarding key sites in the Somali capital where a U.N.-backed interim government and its Ethiopian military allies are fighting Islamist insurgents.

The multinational force, AMISOM, was supposed to be 8,000 strong but has been operating for months with 2,200 soldiers from Uganda and Burundi.

General Salim Ndikumana, acting deputy commander of the Burundian contingent, told Reuters 400 troops landed on Saturday and that about the same number again would be arriving soon.

Last month, Islamist al Shabaab rebels vowed to shoot down aircraft using the coastal airstrip and fired mortar shells at another AU military plane that touched down there on Sept. 19.

The threats effectively shut down the city's main airport until Thursday, when a civilian plane carrying 120 Somali deportees from Saudi Arabia managed to land without incident.

Peacekeepers at the airport and elsewhere in the capital have been targeted in a series of attacks since Islamists launched an Iraq-style insurgency in early 2007 that has killed nearly 10,000 civilians and an unknown number of combatants.

Seven Ugandan peacekeepers and one Burundian have died.

Having ruled south Somalia for six months in 2006, but then been forced out by allied Ethiopian-Somali troops, the Islamists have regrouped and now control large swathes of the south again.

"The Somali government hopes the AU will complete its promise of 8,000 troops soon," said Mohamed Jama Ali, acting permanent secretary at the Foreign Ministry.

The worst insecurity for nearly two decades in the Horn of Africa country has fuelled a wave of kidnappings this year as well as an explosion of piracy in shipping lanes off the coast.

Combined with drought, inflation and high food and fuel prices, the violence has triggered a humanitarian catastrophe that aid workers say is the worst in Africa. (Writing by Daniel Wallis; Editing by Elizabeth Piper)

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