Mashood Adebayo Baderin, working as an expert on Sudanese rights for the United Nations, called on Khartoum to do more to address human rights.
"I note with concern that Darfur conflict-related crimes are currently being tried in the ordinary courts and that human rights officers have been denied access to these trials," he said in a statement.
U.S. officials last week said they were "profoundly concerned" about a recent uptick in violence in the region pitting tribal militias against government forces.
"In just the first month of 2013, more civilians have been newly displaced by violence in Darfur than in all of 2012," said U.S. State Department spokeswoman Victoria Nuland.
Baderin called on all parties to the Darfur conflict to uphold commitments to peace.
The Sudanese government and a rebel faction of the Justice and Equality Movement last month agreed to the preliminary agenda for peace talks.
The United Nations estimates that tribal fighting in the Sudanese region of Darfur left as many as 100 people dead and two dozen villages destroyed in early January.
The International Criminal Court in 2009 issued an arrest warrant for Sudanese President Omar al-Bashir on war crimes charges in Darfur.