By Abdi Sheikh and Feisal Omar

MOGADISHU (Reuters) - Islamist militant group al Shabaab on Sunday seized control of yet another town in central Somalia after it was abandoned by African Union peacekeepers, a militant spokesman and a local official said, the third to fall to insurgents this month.

On Sunday, an Ethiopian contingent abandoned the town of Halgan in the Hiran region, allowing the group's fighters to enter soon after, said Abdiasis Abu Musab, al Shabaab's spokesman on military operations.

A military offensive launched in 2014 by AU forces and the Somali army pushed out of major strategic centers, but the insurgents, who once held sway over much of the Horn of Africa country, still control some settlements and rural areas.

The fall of Halgan was confirmed by Dahir Amin Jesow, a member of parliament from the region, who said residents are being subjected to reprisals at the hands of the insurgents.

"Each day, civilians are being beheaded over suspicion that they are government supporters," he told Reuters. "We do not have a government that is effective enough to protect our civilians."

The reason for the peacekeepers' withdrawal was not clear. Officials in Ethiopia were not immediately available for comment.

Somalia has been convulsed by instability, conflict and lawlessness since the early 1990s following the toppling of military dictator Mohamed Siad Barre.

Propped up by the African Union-mandated force known as AMISOM, Somalia's military and central government have strengthened their grip on the country but a relentless campaign of violence by al Shabaab persists.

The group regularly attacks AMISOM's troops, which is made up of about 22,000 soldiers and police from African nations supporting Somalia's government and army.

Al Shabaab aims to drive out the peacekeepers, topple Somalia's Western-backed government and impose a strict version of Islam.

(Writing by Aaron Maasho; Editing by Raissa Kasolowsky)