"We have in place contingent measures to ensure that areas in Bay and Bakool...remain stable and secure in the event of further Ethiopian troop withdrawals," said Andrew Gutti, commander of African Union Mission for Somalia (AMISOM), referring to southwest Somali regions currently controlled by Ethiopia.
Ethiopian troops, the strongest military power in Somalia's southwest ever since their November 2011 invasion, pulled out of the town of Hudur on Sunday, the capital of Bakool region.
Hours later, Somalia's Al-Qaeda-linked Shebab swept into the town, their most important territorial victory for over a year.
The capture prompted jubilant celebrations, which included the beheading of an influential cleric in the town.
Despite a string of losses in recent months, the Shebab remain a potent threat, still controlling rural areas as well as carrying out guerrilla attacks in areas apparently under government control.
Somali militia forces allied to the Ethiopians, as well as a column of some 2,000 terrified civilians, fled shortly after the Ethiopian pullout.
Security sources say the withdrawal from Hudur could signal a wider pullout of Ethiopian forces including from the key city of Baidoa, warning that if this happens, the 17,000-strong AMISOM would be hugely overstretched.
AMISOM, which fights alongside Somali government forces, "is closely monitoring developments following the withdrawal of Ethiopian troops from Hudur," the force said in a statement.
"We are conducting a review of our troop deployments...and remain confident that there will be sufficient coverage," Gutti added.
Security sources say that AMISOM would struggle at its current capacity to take over Ethiopian positions, while Somali troops who have worked closely with Ethiopian troops might not necessarily cooperate so well with other forces.
So far, Hudur is the only major town Ethiopians have pulled out of, but troops are also packing kit in Baidoa in apparent preparation to leave.
Ethiopian troops entered Somalia in late 2011 to attack Shebab bases, shortly after Kenyan troops invaded Somalia from the far south.
While Kenyan troops have since joined AMISOM, Ethiopia remained separate.