Somalia’s Shebab extremists killed at least six UN workers on Monday when they set off a huge bomb which ripped through a staff bus in the northeastern town of Garowe, police said.

Somalia's Shebab extremists killed at least six UN workers on Monday when they set off a huge bomb which ripped through a staff bus in the northeastern town of Garowe, police said.

Four of those killed worked for the UN children's agency, Unicef, while four other Unicef staff were in a "serious condition", the agency said in a statement.

The head of the United Nations in Somalia, Nick Kay, said he was "shocked and appalled" by the loss of life, while Somali President Hassan Sheikh Mohamud condemned a "brutal attack".

Somalia's Al-Qaeda-affiliated Shebab insurgents claimed responsibility for the attack, branding the United Nations a "colonization force in Somalia".

Local police chief Ahmed Abdulahi Samatar said four of those killed were foreigners and two were Somalis, while seven others were also wounded, two of them foreigners.

"In attacking Unicef, Al-Shebab has also attacked Somali children," Mohamud added. "It is an attack against the future of our country and I condemn it in the strongest possible terms."

The minibus, marked with the UN logo, was ripped apart by a ferocious blast.

"The improvised explosive device attack occurred when the staff were travelling from their guest house to the office, normally a three-minute drive," Unicef said in a statement.

No details of nationalities of the foreigners killed and wounded were given.

Garowe, in the northeastern region of Somalia, is capital of the semi-autonomous Puntland region.