Uganda will send a 410-strong special force to guard UN installations in Somalia’s capital, Mogadishu, a Ugandan army spokesman told BBC.
The protection squad will free up thousands of UN-backed troops to pursue militant Islamists in the city, Paddy Ankunda said.
The al-Qaeda-linked al-Shabab group has stepped up attacks in Mogadishu in recent weeks.
At least 12 people were killed in a suicide bombing last Thursday.
Last June, it carried out a major assault on the main UN base in Mogadishu, leaving at least 22 people dead.
On Tuesday, a Somali military general, Hassan Mohamud, assumed the post of mayor of the city, following the removal of the incumbent, Mohamed Nur, who is known by his nickname of Tarzan.
The government’s decision to appoint him shows that improving security in Mogadishu remains its key priority, correspondents say.
A 22,000-strong African Union (AU) force, operating under a UN mandate, is battling al-Shabab in Somalia.
Col Ankunda said the protection squad would ensure the AU force was not “bogged down” escorting UN staff – many of whom are involved in aid work.
“Amisom [the African force] will be freed to follow al-Shabab wherever they are hiding,” he added.
Al-Shabab lost control of most of Mogadishu in 2011 to AU and Somali government troops.
It has changed its strategy since then, launching guerrilla-style attacks – including suicide bombings and night-time mortar raids.
Last month, al-Shabab fighters stormed Villa Somalia, the seat of government in Mogadishu, killing at least 11 people.
The group has waged an eight-year insurgency to overthrow the weak UN-backed government and create an Islamic state in Somalia.