The Janjaweed militia in Darfur are fighting with direct support and orders from Sudan's government, a man claiming to be a former member has told the BBC.
The Janjaweed are accused of 'ethnic cleansing'
"Ali" said he had taken part in attacks on Darfur villages after they had been bombed by the Sudanese air force.
He said he had seen ministers at training camps for the pro-government Arab militia.
Khartoum has always denied any links to the Janjaweed, who have been accused of war crimes against civilians in Darfur.
More than two million people have fled their homes during the three-year conflict.
A man identified only as "Ali" told the BBC's Newsnight programme that Sudanese ministers gave express orders for the activities of his unit, which included rape and killing children.
"The Janjaweed don't make decisions. The orders always come from the government," he said.
"They gave us orders, and they say that after we are trained they will give us guns and ammunition."
"Ali" - who is now seeking asylum in Britain - said the men who had trained them were wearing the uniforms of the Sudanese military, adding that Interior Minister Abdul Rahim Muhammad Hussein was a "regular visitor".
The former fighter said the majority of the victims were civilians, mostly women, and also talked of "many rapes" committed by the Janjaweed.
"Whenever we go into a village and find resistance we kill everyone," he said, but denied that he personally killed or raped civilians.
Hilary Benn, a British government minister who visited Darfur on Monday, said the man's evidence was "clearly very serious".
Mr Benn urged him to speak to investigators from the International Criminal Court.
The conflict began in the arid and impoverished region after a rebel group began attacking government targets, saying the region was being neglected by Khartoum.
The rebels say the government is oppressing black Africans in favour of Arabs.
Khartoum has always denied backing the Arab militias, saying the problems in its rebel Darfur region are being exaggerated for political reasons. President Omar al-Bashir has called them "thieves and gangsters".
After strong international pressure and the threat of sanctions, the government promised to disarm the Janjaweed.
But so far there is little evidence this has happened.