Fresh fighting hit the Somali capital Mogadishu Thursday morning, with violent clashes erupting between Islamist al Shabaab rebels and government forces, the local Mareeg Online news site reported.
More on GlobalPost: Somalia's land of plenty
Residents said heavy fire was exchanged between both sides before dawn in Hilliwa and Karaan districts.
Information on casualties was not immediately released.
The AU forces are understood to have used tanks and heavy artillery, and the sound of weapons could be heard in major parts of the capital.
Thursday's violence comes after al Shabaab on Wednesday attacked a military training camp run by AU troops in Wadajir, south of Mogadishu, in an area considered to have been safe, the BBC reported.
At least five people were killed in a suicide attack on Tuesday in a suicide attack in Mogadishu's Hodan district.
Somalia's transition government declared Mogadishu was under its control in August, when al Shabaab, whose name means “the Youth,” withdrew from the city.
Mukhtaar Robow, one of the group's leaders, said the change came about because “al Shabaab means youth but many of us, including the leaders, are very old.”