Kidnapped men have been forced to work on farmland controlled by Janjaweed militias, a coalition of African charities says.

Eyewitnesses also say the Sudanese army has been involved in abducting women and children to be sex slaves and domestic staff for troops in Khartoum.

But Khartoum said the report was "very naive" and called the authors ignorant.

"The government does not condone abductions and it is not government policy," a government spokesman told the BBC.

"We are working hard to stop such violations. The rebel factions are mostly to blame for abductions in Darfur."

Up to 300,000 people have died since conflict began in Darfur in 2003 and at least 2.7 million people have fled their homes.

Thousands of people from non-Arabic speaking ethnic groups in Darfur have been targeted, says the report, published by the Darfur Consortium on Wednesday.

The group of 50 charities says it has around 100 eyewitness accounts from former abductees.

Victims have been rounded up during joint attacks on villages by the Arabic-speaking Janjaweed and the Sudanese Armed Forces, according to the study.

Civilians are also tortured and killed while their villages are razed to ethnically cleanse areas, which are then repopulated with Arabic-speaking people, including nomads from Chad, Niger, Mali and Cameroon, it says.

Most of the abductees are women and girls, but there is new evidence in Darfur of kidnappers targeting men and boys for forced agricultural labour, says the report.

The abducted women and girls, meanwhile, are raped and forced to marry their captors as well as carry out household chores and sometimes cultivate crops, according to the study.

'Regular beatings'

The report includes the testimony of children forced to become domestic workers.

One boy said he had suffered regular beatings from his Janjaweed abductors.

"They were treating me and the other boys very badly, they kept telling us that we are not human beings and we are here to serve them, I also worked on their farms," he said.

A woman said she was kidnapped from a refugee camp and her captors "used us like their wives in the night and during the day we worked all the time.

"The men they abducted with us were used to look after their livestock. We worked all day, all week with no rest."

Sudan's government has always denied the existence of slavery in the country, although Khartoum has previously admitted abductions occurred in the north-south civil war of 1983-2005, when up to 14,000 people were kidnapped.

But a senior Sudanese politician who did not wanted to be named said kidnappings had also occurred more recently in Darfur.

"The army captured many children and women hiding in the bush outside burnt villages," he told the report's authors.

"They were transported by plane to Khartoum at night and divided up among soldiers as domestic workers and, in some cases, wives."

Call to action

The report urged Sudan's government to disband the Janjaweed and other militia and to fully co-operate with the United Nations and the African Union.

Dismas Nkunda, co-chair of the Darfur Consortium, said: "Urgent action is clearly required to prevent further abductions and associated human rights violations, and to release and assist those who are still being held."

The study also calls for the mandate of the joint United Nations-African Union peacekeeping force in Darfur (Unamid) to be beefed up so it can use force to protect civilians.

The Darfur Consortium also wants Khartoum to prosecute all those responsible for abductions and ban them from holding public office. It notes that no-one has ever been arrested over the wave of kidnappings.