(Adds medical source, paragraph 3)

MOGADISHU, Oct 12 (Reuters) - More Burundian troops deployed in Mogadishu on Sunday to bolster an African Union peace force caught in the middle of a deepening insurgency.

A source at the main airport in Somalia's capital said a further 420 soldiers from Burundi arrived on Sunday, a day after 400 Burundian peacekeepers landed.

Later on Sunday, a roadside bomb hit an AU convoy wounding at least two peacekeepers, witnesses said. An AU medical source said the injured men were among the new arrivals from Burundi.

The AU mission is guarding key sites in the city where a U.N.-backed interim government and its Ethiopian military allies are fighting Islamist rebels and heavily armed clan militias.

The multinational force, AMISOM, was supposed to be 8,000 strong but has been operating for months with just 2,200 soldiers, all from Uganda and Burundi. The weekend's deployments bring the strength of the force to 3,020.

Islamist al Shabaab rebels have said they will shoot down aircraft using the coastal airstrip and fired mortar shells at another AU military plane that touched down there on Sept. 19.

The peacekeepers have been targeted in a string of bombings and ambushes since the Islamists launched their rebellion early last year. The fighting has killed nearly 10,000 civilians. Seven Ugandan soldiers 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 worst insecurity for nearly two decades in the chaotic Horn of Africa country has fuelled a wave of kidnappings this year as well as attacks by pirates offshore.

Combined with drought, inflation and high food and fuel prices, the violence has also triggered a humanitarian disaster. (Reporting by Abdi Sheikh and Ibrahim Mohamed; Writing by Daniel Wallis; Editing by Angus MacSwan)

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