Al-Qaeda linked Shebab fighters in Somalia are fleeing their key stronghold of Afgoye ahead of an advancing column of government and African Union troops, military commanders said Thursday.

Sporadic shooting was heard on the third day of an offensive against hardline insurgent positions as the joint force of AU and Somali troops closed in on Afgoye town, some 30 kilometres (18 miles) northwest of the capital Mogadishu.

Thousands of impoverished civilians living in plastic and rag hut shelters along the Afgoye corridor -- the main road from Mogadishu to Afgoye, and the world's largest concentration of displaced people -- are fleeing fearing violence.

"We are few kilometers (miles) away from Afgoye and, God willing, we will complete the mission very soon," said Somali military commander General Abdulahi Osman, who is with the advancing government and AU troops across the arid plains.

"We are not encountering much resistance and so far the enemy is fleeing," he added.

The extremist Shebab have claimed to have repulsed the attack and have boasted of killing several soldiers.

More than 400,000 people, around one third of all the displaced people in Somalia, were living in the Afgoye corridor at the start of the year, fleeing war or drought, according to the UN, which has warned civilians must be protected.

"It is as if no one will be left in the Afgoye corridor today, hundreds of families are returning to Mogadishu before the fighting reaches them," said Abdirahman Ahmed, a father of four, after fleeing into Mogadishu.

Long lines of trucks and buses piled high with people and their belongings lined the road towards Mogadishu, defying AU calls for civilians to remain in their homes.

However, army commanders said they were advancing on foot and avoiding the main road of the Afgoye corridor to allow civilians to leave the battle zone.

"The army is advancing with caution, and not aiming directly for the main tarmac road to give civilians access to leave amid the fighting," said Osman.

Afgoye is a strategic town that commands a road junction for routes to the north, west and south of Somalia, and its loss to Shebab would be another major blow for the group, who have been on the backfoot for several months.