GENEVA, April 6 (Reuters) - The U.N. human rights chief on Friday demanded Sudan accept an independent investigation into reported rape and sexual violence during an attack by government forces and their militia allies in Darfur last December.

In the latest in a series of reports on violence in Sudan's huge western region, the High Commissioner for Human Rights Louise Arbour said the 15 victims of the incidents included a 13-year-old girl and two pregnant women.

"The High Commissioner is seriously concerned that rape and other sexual violence ... was used as a weapon of war to cause humiliation and instil fear," her office said in a statement.

An independent and impartial body, trusted by all sides and including women investigators, should be set up, its findings made public and those responsible brought to trial, the statement added.

The attacks were carried out in mid and late December in eastern Jebel Marra in central Darfur by "Sudanese government forces and allied militia", Arbour's office said.

Khartoum denies responsibility and points the finger instead at rebel groups that refused a 2006 peace deal.

The United Nations says over 200,000 have died as a result of violence in Darfur since 2003 when a simmering ethnic conflict erupted into fighting, with the government's militia allies blamed for some of the worst atrocities, including rape and murder.

In a separate report, Arbour also demanded that former rebel chief Minni Arcua Minnawi say what happened to 19 men detained by his forces last September in the Darfur town of Gereida and who have not been seen since.

Minnawi, whose Sudan Liberation Movement (SLM) signed a peace deal with the government last May, is senior assistant to President Omar Hassan al-Bashir with special responsibilities for Darfur.

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