African Union forces, backed by Kenyan and Ethiopian troops, in recent weeks took control of parts of Somalia once held by al-Shabaab, an al-Qaida affiliate in control of areas outside Mogadishu.
Abdullahi Shirwa, head of Somalia's National Disaster Management Agency, told the United Nations' humanitarian news agency IRIN that Mogadishu has seen a massive influx of civilians.
"Since Feb. 18, we estimate that between 10,000 and 11,000 families have arrived in Mogadishu, fleeing fighting or the fear of fighting in their home areas," he said.
One resident who spoke with IRIN said al-Shabaab is forcing residents outside of Mogadishu to fight against pro-government forces.
Airstrikes hit an area controlled by al-Shabaab last week, killing four people. Correspondents working for the BBC reported the blast was bigger than anything likely carried out by African forces fighting in the region.
There hasn't been a functioning central government in Somalia since the 1990s. World leaders at a London conference for Somalia last week pledged support for the fledgling government.