African Union-backed Somali government troops attacked hardline Shebab insurgent positions at dawn on Friday in the northeast of the war-torn capital Mogadishu, officials said.

An artillery barrage first poured down on the district -- which AU forces claim was already largely abandoned by civilians -- before tanks and ground troops moved into the area to attack rebel positions.

"We have advanced on their last positions in Suqaholaha district, taking control of key locations," said Somali military commander Abdulahi Ali Anod.

"The operations will continue until we get rid of the terrorist remnants, who are now on the run," he added.

The assault is the latest in a long running offensive to drive Al-Qaeda allied Shebab from holdout positions, after the majority of insurgents left fixed defences in Mogadishu last year and switched to guerrilla attacks.

The Suqaholaha neighborhood is the main Shebab stronghold left in the anarchic capital, and the attack comes two weeks after AU troops launched a similar offensive in southern Mogadishu.

"It will extend the city's defences and deny the Al-Qaeda backed terrorists important ground from which they have been targeting the city's population," the African Union mission in Somalia (AMISOM) said in a statement.

Extremist Shebab gunmen have been fighting to topple the weak Western-backed Somali government in Mogadishu, where the administration survives under the protection of the 10,000-strong AU force.

"There was heavy shelling by the AMISOM peacekeepers this morning, and now we see their tanks and infantry have entered the area. I saw three injured soldiers carried on a pickup truck," said Abdulahi Nuradin, a witness.

"The Somali troops and their allied AMISOM forces are advancing onto Suqaholaha. There was heavy fighting this morning but now the sound of the gunfire is going down," said Ahmed Yare, another witness.