Heavy fighting has erupted in Somalia's capital, as pro-government forces try oust militant group al-Shabab from its last strongholds in the city.

Residents of northern Mogadishu reported hearing explosions and gunfire Monday as Somali government and African Union soldiers advanced on al-Shabab positions.

The offensive comes several days after al-Shabab suicide bombers killed at least 80 people at a Somali government compound.

It was al-Shabab's first major attack in the city since the group withdrew most of its fighters from Mogadishu in August. The group has since maintained control of a few outlying districts of the capital.

Al-Shabab once controlled nearly all of Mogadishu but has steadily lost ground over the past year to AU and government forces.

The militants seek to topple the U.N.-backed transitional government and impose a strict form of Islamic law across Somalia.

The AU force, technically a peacekeeping mission, consists of about 9,000 soldiers from Uganda and Burundi.