The first African Union (AU) peacekeepers have arrived in the Somali capital, Mogadishu, officials say.
There is a fear the force could be under strength and under-resourced
They are the first of a 1,700-strong force promised by Uganda's government.
So far, the AU has managed to raise only about half of the required 8,000 troops. Burundi, Nigeria, Malawi and Ghana are also expected to contribute.
The force takes over from Ethiopian troops who intervened to help Somalia's transitional government oust Islamist forces from the capital late last year.
The head of the AU delegation, Geoffrey Mugunya, said on Monday that the mission would not interfere in Somalia's internal affairs, but support the transitional government's own efforts to train enough security forces.
The BBC's Karen Allen in Uganda says the fear is that, just like Darfur, the force could be under strength and under-resourced.
Any targeting of Ugandan troops by insurgents in Somalia could give other troop-contributing nations cold feet, derailing the mission, our correspondent adds.