Mogadishu: At least six UN workers have been killed in Somalia by a huge bomb placed by al-Shabab militants that destroyed a bus.

The head of the United Nations in Somalia, Nick Kay, has condemned the attack in the northeastern town of Garowe and said he was "shocked and appalled by (the) loss of life".

"We targeted the UN in Garowe, we killed some and wounded others": Shebab spokesman Abdulaziz Abu Musab. Credit:AP

Somalia's al-Qaeda-affiliated al-Shabab insurgents claimed responsibility for the attack, branding the United Nations a "colonisation force in Somalia".

"We have confirmed the death of six UN staff, including a foreign national," Somali police official Abdullahi Mohamed said.