Suspected members of Somalia's al-Qaeda-affiliated al-Shabab fighters have attacked a fortified area housing regional government headquarters, an airport and a United Nations compound in the central town of Baidoa, security officials said.

"They are Shebab disguised in Somali military uniforms. That's how they managed to enter," a Somali police official in Baidoa, Mohamed Dahir, told AFP by telephone on Thursday.

Officials said around four gunmen shot and wounded or killed several people at the gate of the fortified area before entering it.

Two of the attackers are thought to have detonated bombs strapped onto them, a security official said.