Somali militant group al-Shabab has banned an aid group from the shrinking but still significant area under its control.

The militants said in a series of Twitter messages Monday that it has revoked permits for London-based Islamic Relief. Al-Shabab said the aid group was covertly working with other organizations it has banned, especially the United Nations World Food Program.

After losing the port city of Kismayo last week, al-Shabab no longer controls any major towns in Somalia, but it continues to hold rural areas and roads that connect population centers.

African Union forces fighting the militants said Sunday that they and Somali government troops had captured the town of Wanla Weyn from al-Shabab and secured a nearby airbase.

The AU force said it has begun an offensive aimed at clearing al-Shabab from the area between Afgooye, west of Mogadishu, and the town of Baidoa.

Al-Shabab, which has an alliance with al-Qaida, once controlled most of southern and central Somalia and imposed a strict version of Islamic law on the population. But over the past two years, the group has lost most of its territory to combined AU, Kenyan, Ethiopian and Somali government forces.