At least 45 African Union and Somali national army forces died in an al Shabaab raid on their base in southern Somalia on Tuesday, western sources told CNN.
More than 100 troops were unaccounted for.
At least 20 of the troops were Ugandans taking part in the mission to hold a base in Janaale, Somalia, the sources said Wednesday.
Al Shabaab has been designated a terrorist organization by several countries, including the United States. It has waged a long insurgency in Somalia.
The extremist group has stepped up hit-and-run raids on remote bases as a tactic to fight the African Union forces seeking to break their insurgency.