African Union (AU) troops raped women and girls seeking medical aid or water from their bases in Somalia's capital, Mogadishu, a rights group says.
The troops had "misused" their power over women fleeing violence and poverty, Human Rights Watch (HRW) said.
One Muslim girl, aged 15, reportedly had her headscarf ripped off before being raped.
The AU, which has some 22,000 troops fighting militant Islamists in Somalia, says it will investigate the claims.
In 2012, the UN recorded 1,700 rapes in camps for displaced people in Somalia.
Many were thought to have been carried out by members of the Somali security forces.
Last year, there was a huge public outcry following claims that AU and government soldiers had gang-raped a woman in Mogadishu.