Women and young girls living in refugee camps in Somalia after fleeing violence at home were raped by African Union troops, a human rights group says.

The female refugees were seeking medical aid and water from the African Union bases in the capital Mogadishu when they were attacked, members of Human Rights Watch told the BBC.

The troops "misused" their power over the women. One Muslim girl, 15, went to an AU base of Burundian soldiers to collect medicine for her sick mother when she was assaulted.

"First he ripped off my hijab [headscarf] and then he attacked me," HRW quoted the victim as saying.

A Burundian soldier gave the girl $10 (£6) after raping her, HRW said.

The African Union says it will investigate the claims. AU forces-- deployed in 2007 with most of its 22,000 troops coming from Uganda and Burundi—have been fighting militant Islamists in Somalia.

Most of the victims lived in camps for people who escaped violence and the 2011 famine, HRW said.

"The AU can no longer turn a blind eye to the abuses on Amisom [AU Mission in Somalia] bases, as it's undermining the very credibility of the mission," said Liesl Gerntholtz, HRW's women's rights director.

The group interviewed 21 women and girls-- some as young as 12-- who described sexual exploitation and rape by Ugandan or Burundian soldiers in the AU force, HRW said.

Only one child rape case is before Uganda's military court in Kampala, it added.

"Some Amisom soldiers have used humanitarian assistance, provided by the mission, to coerce vulnerable women and girls into sexual activity," HRW said.

The United Nations recorded 1,700 rapes in camps for displaced people in Somalia in 2012. Many were believed to have been carried out by members of the Somali security forces.

Meanwhile, at least 12 civilians were killed Monday in a suicide attack targeting African Union troops in Somalia's Lower Shabelle region, the first serious assault by suspected Islamic extremists after the killing of al-Shabab's top leader in a U.S. airstrike last week, police and a regional official said.

A suicide bomber detonated his explosives-laden car next to a convoy of African Union forces moving near two minibuses, said Somali police official Hassan Ali. Then, amid the confusion, another bomb went off when a second suicide attacker rammed his car into a convoy. There were no fatalities from the second blast, Ali said.

Al-Shabab claimed responsibility for the attack in a radio broadcast. Al-Shabab said in a statement over the weekend that it remains aligned with al Qaeda, according to the SITE Intelligence Group, which monitors statements by Islamic militant groups.

The U.S. State Department declared al-Shabab a terrorist organization in February 2008.

The Associated Press contributed to this report.