At least 12 people, including four Americans, were killed and 27 others wounded when a suicide bomber rammed a vehicle packed with explosives into an African Union convoy southwest of the Somali capital Mogadishu.

The attack, the latest in a wave of violence, comes exactly one week after a US air strike killed the chief of the al-Qaeda-linked al-Shabab group, prompting threats of retaliation from the group.

Speaking to Al Jazeera, al-Shabab's military operations spokesman claimed responsibility for the attack that took place near the town of Afgoye, about 30km northwest from Mogadishu.

Two soldiers from AMISOM - the African Union Mission in Somalia - were wounded in the attack, the local governor said.

'Americans targeted'

The al-Shabab spokesman said the target of the attack was an American commander who trains special forces in Mogadishu.