Uganda's military, which forms the backbone of the African Union peacekeeping force in Somalia, has deployed an air force contingent to reinforce troops who hope to retake a city from al Qaeda-linked militants before the government's mandate expires.

A combined force in Somalia is planning an onslaught on Kismayu, Somalia's second biggest city which is a hub for the al Shabaab Islamist militants, before August 20.

Weakened by internal divisions and financial constraints, the rebels have surrendered territory in Mogadishu, central and southern Somalia where they are also battling Ethiopian forces as well as Kenyan soldiers now integrated into the African Union mission AMISOM, Reuters reports.

A U.S.-backed plan calls for Somalia to establish a legitimate government accepted by fractious clans and a new parliament and constituent assembly to replace institutions plagued by corruption and infighting.

The National Constituent Assembly, sitting in Mogadishu for the last week, approved a provisional constitution to replace an 8-year-old Transitional Federal Charter and lead to the end of the transition process on August 20, when the U.N.-backed government's mandate expires.

Lt. Moses Omayo, spokesman for the Uganda Airforce, told Reuters the contingent comprising transport and combat helicopters left Uganda for Somalia on Monday.

"Conditions keep changing in Somalia and we felt it was the right time to deploy an air component to support our ground troops," Omayo said.

"We have sent helicopters which will provide air cover for combat troops, escort convoys, conduct rescue missions and airdrop forces," he said.

Omayo said the helicopters would conduct operations only in areas in which troops that include those from Burundi and Djibouti are deployed.

The rebel group has waged a five-year campaign to topple Somalia's Western-backed government and impose its harsh interpretation of Islamic law.

U.S. Secretary of State Hillary Clinton last week thanked President Yoweri Museveni of Uganda for helping in Somalia to fight the al Shabaab insurgents.