The African Transitional Mission in Somalia (ATMIS) said it has completed the first phase of a troop reduction aimed at eventually putting security fully in the hands of the national army and police.
It said in a statement that a total of seven bases had been handed over to Somali security forces, enabling the drawdown of 2,000 troops by the June 30 deadline.
ATMIS chief logistics officer Bosco Sibondavyi described the handover as an "important milestone" in the implementation of the Somalia Transition Plan and UN Security Council resolutions on the transfer of security responsibility, AFP reported.
The Security Council on Tuesday renewed for six months its authorization of the AU force, which has a deadline of the end of September for the departure of a further 3,000 soldiers.
The ATMIS contingent had included over 19,000 soldiers and police officers from several African nations including Burundi, Ethiopia, Kenya and Uganda, but will have to be reduced to zero by the end of 2024.
In April 2022, the Council approved the replacement of AMISOM (African Union Mission in Somalia), which had been set up in 2007, by ATMIS, a mission with a reinforced mandate to fight Al-Shabaab militants.
The group, which has links with Al-Qaeda, has been waging a bloody insurgency against the fragile internationally backed government in Mogadishu for more than 15 years.
Its fighters continue to carry out deadly attacks despite a major offensive launched last August by pro-government forces, backed by the AU force and US air strikes.
In the deadliest most recent attack, 54 Ugandan peacekeepers were killed in late May when Al-Shabaab militants stormed an AU base southwest of the capital.