Al-Shabab attacked pro-government forces on Kudhaa island before dawn Saturday, seizing control of it from the Interim Juba Administration (IJA).
Residents tell VOA that al-Shabab displayed 43 bodies of IJA fighters on the beach after the fighting and later executed nine IJA members and eight civilians accused of working with the administration.
Sources tell VOA that al-Shabab lost 31 men in the fighting, bringing the death toll to 91.
Kudhaa, located 130 kilometers southwest of Kismayo, was captured from al-Shabab just two weeks ago by pro-government forces. The militants had used the island to export charcoal, a key source of revenue for the group.
Pro-al-Shabab media have reported that the militants seized large quantities of weapons in Saturday's fighting, including mortars and machine guns.
An IJA spokesman, Abdinasir Serar, has vowed that his forces will recapture Kudhaa.
Al-Shabab once controlled most of central and southern Somalia but has steadily lost ground in recent years to government and African Union forces. The al-Qaida-linked group remains dangerous, and has carried out two major attacks this year on the presidential palace in Mogadishu.