Sudan has cast doubts on the authenticity of documents which allegedly show that the government is backing Arab militias in Darfur.
Even in Chad, refugees are not safe from Janjaweed attacks
The Janjaweed are accused of killing thousands and using mass rape against non-Arabic groups.
New York-based Human Rights Watch says it has government documents showing that officials directed recruitment, arming and support of the Janjaweed.
But Foreign Minister Mustafa Ismail said the documents were "90% false".
"It is easy to go to a typist and put a [letter] heading," he told the BBC's Newshour programme.
"The Sudanese government... never armed or encouraged the Janjaweed," he said.
But he said there were other, private militias in the area, set up to protect civilians against attacks by Darfur's rebel groups.
Last week, one of the alleged Janjaweed leaders, Musa Hilal, said that he had raised a self-defence militia in response to government calls.
He denied claims that his forces had been responsible for ethnic cleansing".
Sudan says about 6,000 police are being sent to protect the refugees, following reports of widespread rape of women in the camps by the militias.
Aid agencies warn that thousands more of the one million displaced people could die from hunger and disease.
A group of British charities - known as the Disasters Emergency Committee - is launching an urgent aid appeal for the people of Sudan on Tuesday.
Human Rights Watch said it had obtained four documents from an undisclosed source, signed by government authorities.
One from the deputy interior minister is alleged to ask for the recruitment of "knights" - a reference to militia.
The Janjaweed often fight on horseback, or use camels
Group director Kenneth Roth said the documents showed that the Sudanese government had lied to US Secretary of State Colin Powell and UN Secretary General Kofi Annan when it denied recruiting and arming the militias.
"We can no longer trust Khartoum to police itself when Khartoum is part of a large problem," Mr Roth said.
Mr Annan, who recently visited Darfur, said the international community should insist the Sudanese government live up to its commitments.
A separate UN report said militias were continuing to carry out attacks in the region.
The deployment of extra security forces to Darfur's refugee camps follows international pressure for the government to prevent rape and abuse by the militias.
The security measures were agreed by Sudan after talks with Mr Annan earlier this month.
But the BBC's Hilary Andersson in Darfur says many in the camps do not trust the police because they believe the Sudanese authorities have been backing the militias.
And our correspondent says insecurity is only part of the problem in the camps, which are filthy and overcrowded.
There is little proper shelter, disease is spreading - and every day new graves are dug for those who have died of starvation, she says.