Militant Islamist rebels in Somalia are banning an aid group from operating in rural areas, despite the residents' urgent need for food.

Al-Shabab said on Twitter Monday that it had banned the British-registered aid group Islamic Relief. The extremists accused the group of working with the United Nations' World Food Program, which the militants will not allow in its territory.

Al-Shabab has been forced out of Mogadishu, Kismayo and many smaller towns but the group still controls rural areas of south-central Somalia. It imposes strict Shariah law on Somalis who live there and allow only a few outside organizations to operate in its area.

The aid group Oxfam, said Monday that water and food shortages in southern Somalia are at critical levels and likely to worsen in coming months.