An independent UN human rights expert said human rights activists and aid workers are being arrested and tortured in Sudan.
Sima Samar, the UN special investigator for Sudan, said the situation there is "critical".
She said the government is also increasingly clamping down on journalists since the International Criminal Court issued an arrest warrant for Sudan's president.
Omar al-Bashir is accused of war crimes and crimes against humanity including murder, extermination, torture and rape in Darfur.
Samar said in a report to the UN Human Rights Council on Tuesday that government forces are still attacking civilians in Darfur.
Sudan is rallying African countries in Geneva this week to prevent Samar's mandate from being renewed.