NAIROBI (AFP) - Internationally funded African Union (AU) troops in war-torn and impoverished Somalia have raped women and traded food aid for sex, Human Rights Watch (HRW) said in a damning report on Monday.

"Some of the women who were raped said that the soldiers gave them food or money afterwards in an apparent attempt to frame the assault as transactional sex," the HRW report said.

There was no immediate reaction from the AU force, AMISOM, a 22,000-strong force with soldiers from six nations, which has been fighting alongside government troops against the Al-Qaeda-linked Shebab insurgents since 2007. AMISOM donors include the UN, European Union, Britain and United States.

The vulnerable women largely came from basic camps in the capital Mogadishu, after fleeing rural Somalia during a devastating famine in 2011.

The AU soldiers, "relying on Somali intermediaries, have used a range of tactics, including humanitarian aid, to coerce vulnerable women and girls into sexual activity", the report read, based on testimonies of 21 women and girls.

"They have also raped or otherwise sexually assaulted women who were seeking medical assistance or water at AMISOM bases."

The youngest interviewed was aged just 12, who said she was raped by a Ugandan soldier.

Several of the women described how they had gone to the AU camp seeking medicine for their sick babies.

"The findings raise serious concerns about abuses by AMISOM soldiers against Somali women and girls that suggest a much larger problem," HRW added.

Only in two cases had the women who spoke to HRW filed police complaints, because they "feared stigma, reprisals from family, police, and the Islamist insurgent group Al-Shebab".

The cases investigated by HRW involved troops from Burundi and Uganda.