By Abdi Sheikh and Feisal Omar
MOGADISHU (Reuters) - Somalia's Islamist rebel group bombed African peacekeepers and government vehicles on Monday in twin attacks that left at least 12 civilians dead, marking al Shabaab's first major attack since promising revenge for the killing of its leader last week.
The blasts within an hour of each other on the same road southwest of Mogadishu targeted a convoy of African Union (AU) troops and a Somali government convoy of police and national security forces. Civilian buses near the first blast were hit.
"We are behind the two car bombs driven by mujahideen (fighters)," Sheikh Abdiasis Abu Musab, spokesman for al Shabaab's military operations, told Reuters.
The attacks were a reminder that al Shabaab, which wants to impose its strict version of Islam on Somalia, remains a potent threat capable of launching major attacks, even after losing its leader Ahmed Godane in a U.S. strike last week.
A senior leader with al Shabaab issued a new set of threats later on Monday, promising revenge on the United States, as well as Kenya and Uganda - two of the countries that have contributed troops to the African Union force in Somalia.
"If you think jihad will stop after killing men, we say, that is a lie," Fuad Mohamed Khalaf Shongole said in an audio recording. "You non-believer Obama, we tell you now is the time for war for the sake of God."
He also vowed to capture Kampala and Nairobi, east African cities that have been targeted by al Shabaab before.
The militants carried out attacks in the Ugandan capital of Kampala in 2010, leaving left 74 dead and dozens more injured. Nearly a year ago, the group carried out a dramatic attack on the upscale Westgate shopping mall in Nairobi that killed 67 people.
Experts had warned Godane's death may lead to more attacks. On Sunday, Somalia's national security minister put the armed forces on higher alert and said the government was preparing for possible attacks on hospitals or schools.
Al Shabaab appointed a new leader at the weekend and said the group's enemies would reap "bitter fruits" of revenge.
Abdikadir Mohamed Sidi, governor of the Lower Shabelle region south of Mogadishu, told Reuters by telephone he was driving behind the AU convoy at the time of the first blast and saw three civilian vehicles in the same area.
He said more than 12 people were killed in one minibus. Two African Union soldiers were injured in that attack, about 20 km (13 miles) from the capital.
The rebels said four American soldiers and a South African soldier were among those killed in the attack on the convoy. But a U.S. government official said no U.S. military forces were involved.
"It is a disaster. The flesh of the people was mixed up and stuck to the tarmac road and the debris of the vehicle," said Major Hussein Ahmed, a senior police officer, describing the scene near the African Union convoy where minibuses were hit.
More than two dozen people were wounded in the first blast, including two African soldiers, the governor said. At least two people were injured in the second blast, said major Hussein Ahmed.
A Reuters witness saw an AU vehicle being towed and the burned remains of passengers inside one of the minibuses.
Al Shabaab, which emerged as a fighting force in 2006, lost control of the Somali capital to African forces in 2011 but has continued hit-and-run gun and bomb attacks. The African Union launched a new offensive this year to retake territory in al Shabaab strongholds in southern and central Somalia.
Following Godane's death last Monday, rebels pledged allegiance to their new leader, the little-known Sheikh Ahmed Umar Abu Ubaida, and reaffirmed its affiliation to al Qaeda.
(Reporting by Edith Honan; Editing by Sonya Hepinstall)