"The bodies were beyond recognition. There was blood and flesh everywhere," Hassan Mohamoud, a witness, said.
Last week al-Shabab launched two suicide car bombs on the main AU force base in Mogadishu, killing 17 peacekeepers.
It was the deadliest single attack on the force of 5,000 troops from Burundi and Uganda since they arrived in 2007.
At least 29 Burundian soldiers have been killed in the country since their mission began.
Al-Shabab said that attack was in retaliation for a US raid on September 14 that killed Saleh Ali Saleh Nabhan, an al-Qaeda suspect, in southern Somalia.
The US military has launched several air attacks inside Somalia in the past against individuals blamed for the bombings of the US embassies in Kenya and Tanzania in 1998.
In May last year, US aircraft killed Aden Hashi Ayro, the then leader of al-Shabab and allegedly a senior al-Qaeda member, in an attack on the central town of Dusamareb.
Al-Shabab - meaning "the Youth" in Arabic - is believed to be largest group among several Islamist and clan militias battling the UN-backed transitional government in Somalia.
Al-Shabab says it seeks to impose its own strict version of Islamic law across Somalia.
The group is accused by the US of having links to al-Qaeda, and is believed to have been reinforced with foreign fighters.
The FBI has expressed concern that al-Shabab may be expanding its reach and actively recruiting Western nationals to fight in Somalia.
No one knows for sure where the group gets its financial and logistical support, but Eritrea and some Arab countries have been accused of funding the conflict in the Horn of Africa.
The country has been ravaged by violence and anarchy since regional commanders overthrew Mohamed Siad Barre, the then president, in 1991, before turning on each other.
Piracy has flourished off the Somali coast, making the Gulf of Aden one of the most dangerous waterways in the world.