The 400-strong "defensive" guard force of Ugandan troops, based at the heavily fortified Mogadishu airport, is "mandated to protect UN staff and installations" in the capital.
Despite the government's insistence that it has improved security, the Shebab continue to carry out bombings -- including targeting aid workers -- in their bid to overthrow Somalia's internationally backed but fragile leadership.
Shebab suicide commandos last June blasted their way into a fortified UN base in central Mogadishu, killing 16 people.
Ugandan troops are already in Somalia as a key part of the UN-mandated African Union force AMISOM, which is fighting the Shebab.
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The insurgents have been driven out of fixed positions in Somalia's major towns by AMISOM troops, but still regularly launch guerrilla raids.
Recent Shebab attacks have targeted key areas of government or the security forces, in an apparent bid to discredit claims by the authorities that they are winning the war against the Islamist fighters.
Continued conflict, compounded by poor rains and funding shortfalls, are threatening the few gains made in Somalia since an extreme famine less than three years ago, with the United Nations and aid agencies warning that the troubled country could be sliding back into a food crisis.