The fighting began, witnesses said Monday, when militants attacked joint Somali and Kenyan military bases on the outskirts of Badade in the Lower Jubba region of southern Somalia, the Shabelle Media Network reported.
Somali and Kenyan forces controlled Badade since militants withdrew a few months ago after a bloodless takeover, a military spokesman said.
SMN also reported residents said a landmine blast hit an Ethiopian convoy in Somalia's central town of Beledweyne.
The blast caused an undetermined number of fatalities among troops, SMN reported. One witness said Ethiopian soldiers fatally shot two civilians after the explosion.
No one had claimed responsibility for the attack.