Human rights campaigners are outraged that a United Nations report alleging grave abuses in western Sudan is being withheld from a UN debate on the issue.
Refugees in Chad have told of arbitrary killings and rape
The international body's Commission on Human Rights is due to discuss the crisis in Darfur, where up to a million people have been displaced.
The debate has been delayed by behind-the-scenes negotiations with the European Union seeking a harsher resolution than African delegates want.
The report, seen by the BBC, details claims of rape, looting and killing by militias with government help.
Meanwhile the government has resumed talks with Darfur rebel groups in Chad.
The report says the atrocities in Darfur "may constitute war crimes and/or crimes against humanity".
It was compiled by a UN team of experts who visited Chad to speak to refugees from the conflict.
It is not being tabled at Thursday's discussions in Geneva, because the UN investigators have just been granted permission by Sudan to visit Darfur itself.
Diplomats said the five-man team was in Nairobi awaiting clearance to continue its task in Darfur.
The report has been delayed until they complete their investigation, but human rights groups are accusing Sudan of a ploy to prevent evidence of atrocities coming under discussion.
Jemera Rone of Human Rights Watch told BBC News Online: "The Sudanese government is playing games with the international community, trying to delay the day of reckoning and prevent any systematic monitoring of its atrocities in Darfur."
The investigators conclude in the report that the Sudanese government has not only allowed militia groups to commit atrocities with impunity, but has worked with them, using its air force to bomb villages and its own troops to drive out the population.
The 53-member Commission on Human Rights is due to vote on a draft resolution by the European Union that refers to "the grave violation of human rights", including "the widespread recourse to rape and other forms of sexual violence, including against children, as a means of
The proposal urges an end to the violence and the appointment of a special UN expert to monitor the situation.
The BBC's Imogen Foulkes says the fact that the commission will not see the UN report is another blow to its credibility, after claims that it has bowed to political pressure from member states.
Talks to bring an end to the conflict resumed in Chad's capital, Ndjamena, on Wednesday.