Conflict in Darfur began in 2003 when mainly non-Arab tribes took up arms against Sudan's Arab-led government, Reuters reported.
Progress on resolving the conflict was a key demand made by the United States before it lifted 20-year sanctions last month that had long-isolated the country.
Sudan said last month that it would extend a unilateral ceasefire with rebels that has been in place since October 2016 until the end of December.
The UN report found that "despite a ceasefire between the Government and various armed opposition groups ... violence against internally displaced people continues to be widespread and impunity for human rights violations persists."
"I urge the Government to address fundamental issues that are preventing the return of displaced people, such as continued violence, including from armed militias, which raise continuing and justifiable fears for their safety and the lack of basic services that leave them dependent on aid," UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said in a statement.
People displaced by the conflict face "outright absence of law enforcement and judicial institutions" and "serious human rights abuses and violations of international humanitarian law," the statement said.