A suicide bomber rammed a vehicle packed with explosives into an African peacekeeping convoy, killing at least two people in the Somali capital Mogadishu, officials have said.

The attack on Friday was claimed by al-Shabab fighters who carried out the deadly assault on a nearby UN base last month and another bombing in a Mogadishu market this week.

No peacekeepers died but a number of people were wounded in the attack.

Ambulance sirens wailed through the congested streets and a plume of black smoke billowed into the sky above the city near the United Nations Development Programme (UNDP) base where 22 people were killed in an al-Shabaab assault last month.

"We have carried eight injured civilians including two women," the director of Mogadishu's ambulance service, Abdikadir Abdirahman, said.

'US officials targeted'

Al-Shabab spokesman Sheikh Abdiasis Abu Musab, told Reuters news agency that the convoy was carrying a number of American officials, but the claim could not be confirmed independently.

"We are behind the martyrdom explosion ... The Americans were our main target," he said.

A diplomatic source, who asked to remain anonymous, said he believed the al-Shabab claim regarding the American presence in the convoy was false.

Al-Shabab, an al-Qaeda-affiliated outfit, was pushed out of bases in Mogadishu by Somali and African forces last year, raising hopes of a return to relative security in a city hit by years of war.

But the fighters have kept up guerrilla-style attacks and continue to control large rural areas, challenging the authority of a government less than one year old.