Militants from Somalia's Al-Qaeda-affiliated Al-Shabaab group rammed a suicide car bomb into an African Union army base early Tuesday and stormed inside, an official and a witness said.
There were no immediate reports on casualties, and the rebels said in a statement that the Amisom troops had fled the base, situated in Janale district, 80 kilometres (50 miles) southwest of Mogadishu in the lower Shabele region.
But Amisom denied claims that the Shabaab had taken over the base.
"Amisom can confirm that the base is still under Amisom control. Reports that the base has been taken over and our weapons captured are false," the AU army force stated in a post on its Twitter account.
The base is thought to be manned by Burundian soldiers.
"There was heavy explosion and fighting broke out at the Amisom base in Janale. We don't have details but we are hearing that Al-Shabaab militants attacked the base," said Mohamed Shire, a Somali military commander based in the region.
A local eyewitness said Al-Shabaab fighters stormed into the base.
"Heavy fighting broke out after a suicide bomber rammed his vehicle into the camp," said local resident Ali Moalim Yusuf. "I saw heavily armed fighters chanting 'Allahu Akbar' ('God is greatest') pouring into the base."
The militants, who are fighting to overthrow Somalia's internationally backed and AU-protected government, confirmed they were behind the attack.
"Mujaheddin fighters captured the base after a suicide bomber struck it, the enemy fled," the Islamists said in a brief statement.
In June, Al-Shabaab fighters killed dozens of Burundian soldiers when they overran an Amisom outpost northwest of the capital. The militants also stage frequent suicide attacks inside the capital.
But Amisom, the 22,000-strong AU force in Somalia, has also made key gains against the militants in recent months, pushing them out of several strongholds in the southwest of the country.