Darfur has been described as the world's worst humanitarian crisis

A report by the US-based group Human Rights Watch details recent cases of rape and assault in the Darfur region.

Up to 50,000 residents have been killed and many more driven from their homes.

The UN also accused the government of using helicopter gunships in Darfur. Sudan denies the charges and insists it is improving security in the region.

The accusations of fresh atrocities come as a senior UN official begins a visit to Sudan's neighbour, Chad, amid concerns about 200,000 Sudanese refugees there.

Civilians continue to be attacked and killed in joint government and Janjaweed militia raids Human Rights Watch Full report The BBC is not responsible for the content of external internet sites

Khartoum has been accused of supporting Janjaweed and other Arab militias blamed for widespread atrocities in the fight against rebels in Darfur.

The UN Security Council last month approved a resolution urging Sudan to put a stop to the violence by the end of August.

The government says it has sent thousands of extra troops to Darfur in an effort to rein in militiamen and protect civilians.

But the report by Human Rights Watch says the government's assurances are not credible.

"In many rural areas and small towns in Darfur, government forces and the Janjaweed militias continue to routinely rape and assault women and girls when they leave the periphery of the camps and towns," it says.

DARFUR CONFLICT 1m displaced Up to 50,000 killed More at risk from disease and starvation Arab militias accused of ethnic cleansing Sudan blames rebels for starting conflict Text: UN Darfur agreement

The document give details of incidents said to have taken place in July.

"Civilians continue to be attacked and killed in joint government and Janjaweed militia raids, particularly in South Darfur," Human Rights Watch says.

It also accuses the government of incorporating militia fighters into the police and other state security forces.

But Sudan's ambassador to Britain, Hassan Abdin, rejected the accusations.

"To talk of governments acting in connivance and collaboration with the Janjaweed is not true," he told the BBC's World Today programme.

He said Sudan was clamping down on the violence and had arrested and tried 100 militiamen.

Humanitarian fears

Earlier the UN charged that the government had carried out bombing raids in southern Darfur.

The UN refugee agency (UNHCR) also says Khartoum is putting pressure on non-Arabs who fled their homes to return, despite concerns over their safety.

Last week, Sudan and the UN agreed on a plan to tackle the crisis, which included setting up safe areas around certain towns and villages.

A UNHCR spokesman in Geneva said many of those had been shot by Janjaweed raiders after returning to their villages.

A senior UNHCR official, Jean-Marie Fakhouri, is due to visit camps for Sudanese refugees in eastern Chad on Thursday.

The BBC's Peter Biles in the region says there is growing concern there about their plight.

Many are without adequate shelter and roads have been closed as a result of recent heavy rains.

Humanitarian relief supplies are unable to reach the biggest camp, Breidjing.

An African Union official says about half of the 300 members of the new African protection force planned for Darfur should be in place by Saturday.