Internationally funded African Union troops in Somalia have gang-raped women and girls as young as 12 and traded food aid for sex, Human Rights Watch, the US-based rights group, says.

The 22,000 AU force, called AMISOM, with soldiers drawn from six nations, have been fighting alongside government troops against the al-Qaeda-linked al-Shabab fighters since 2007.

"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," HRW said in a report on Monday.

AMISOM said the alleged rapes were "isolated" incidents and called the report "unbalanced and unfair".

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

AMISOM donors include the EU and the US.

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.