More than 4,000 Ethiopian troops have formally been absorbed into the African Union force in Somalia.
They will be responsible for security in the south-western regions of Gedo, Bay and Bakool, the AU said.
Ethiopia's contribution takes the AU force to the 22,000-strong level mandated by the UN Security Council.
Ethiopian forces have been operating in neighbouring Somalia for several years, helping the UN-backed government to fight the al-Qaeda-aligned al-Shabab group.
Last year, the UN chief Ban Ki-moon asked for a "surge" of extra troops for the AU force in Somalia, known as Amisom, fearing reversals in advances made over the last few years.
Together with government forces, AmiSom has driven al-Shabab from some key cities, including the capital, Mogadishu, since August 2011.
The BBC says the troops from the Ethiopian army — one of most battle-hardened in Africa - will be based in Baidoa, some 300km (185 miles) north-west of Mogadishu.
