At least six UN workers have been killed in Somalia by a huge bomb placed by Shebab 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".

Somalia's al-Qaeda-affiliated Shebab 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.

"The bomb is believed to have been attached to the minibus and was detonated near the UN office."

Witnesses and security officials suggested the explosion could also have come from a roadside bomb that was detonated as the minibus, which is used to transport staff from a guesthouse to the UN compound, was passing.

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

"Investigations are still ongoing to establish how it happened but I can confirm you that the UN compound was not affected," Mohamed added.

A UN representative in Somalia was unable immediately to confirm the death toll.

Shebab spokesman Abdulaziz Abu Musab confirmed the Islamist group had carried out the attack.

"We targeted the UN in Garowe, we killed some and wounded others. They are part of the colonisation force in Somalia," he said.

The Shebab, meaning "youth", emerged out of bitter insurgency against Ethiopia, whose troops entered Somalia in a 2006 US-backed invasion to topple the Islamic Courts Union that was then controlling the capital Mogadishu.

Shebab rebels continue to stage frequent attacks in their fight to overthrow Somalia's internationally backed government, as well as to counter claims that they are close to defeat due to the loss of territory, regular US drone strikes against their leaders and defections.

They have also carried out revenge attacks across the wider region against countries which contribute troops to the 22,000-strong African Union force in Somalia, AMISOM.

Earlier this month Shebab gunmen attacked Garissa university in the northeast of the country, killing nearly 150 people, mostly students.