He said the push was part of the force's efforts to secure the Somali capital by cutting off the al Shabaab's ability to infiltrate and mount attacks on the city and its people.

The troops took Al Janal hill near the city's main airport as well as the Maslah building that sits on the road to Afgoye, an al Shabaab stronghold, he said.

"Operations are ongoing to expand the zone of security in Mogadishu," Nduwumunsi said in a statement.

However, officials at Al Shabaab denied the claim that their strongholds had been captured by the AU forces, and said they had killed AU soldiers during the fighting.

"We killed 7 Burundians in today's fighting," Sheikh Abdiasis Abu Musab, the spokesman for al Shabaab's military operation, told Reuters. "AMISOM is lying, they have not captured our positions," he added.

Al Shabaab, which wants to impose its harsh interpretation of sharia, the Islamic moral and legal code, relinquished control of the coastal capital in August, under pressure from AMISOM.

The Islamist fighters have since then shifted to towns outside the capital and switched their tactics to guerrilla-style warfare, using roadside bombs, grenade attacks and hit-and-run raids against government installations in Mogadishu.