Some 2m people rely on food aid in war-torn Darfur
United Nations High Commissioner for Refugees Antonio Guterres begins a visit to Sudan and Chad on Monday.
During the 10-day trip he will visit UN camps holding some of around two million civilians displaced by fighting in the western region of Darfur.
Most of the displaced people remain in Sudan. The UN says many are at risk of being attacked or raped by the Sudanese military and its militia allies.
Mr Guterres is expected to raise these concerns in talks with the government.
Across Sudan and in the countries around it, millions of Sudanese still wait for the day they can go home.
Over four million people remain displaced in the wake of the conflict in southern Sudan, despite a recently signed peace deal.
The UN refugee agency is caring for 200,000 people in 12 camps in Chad, who fled the conflict in Darfur. Many have been in the camps for almost two years.
Within Sudan, at least 1.5 million people have fled their homes because of the fighting in Darfur.
The UN refugee agency describes them as having serious protection needs.
Women and girls in Darfur were regularly being raped by militia forces and members of the Sudanese military, according to a UN report published earlier this year.
Mr Guterres will visit the capital Khartoum for talks with government officials, before travelling to Darfur and the UN camps in Chad.
Then he will move to southern Sudan, where the UN hopes that it will be able to begin the return of the millions displaced by the region's long conflict.
Mr Guterres will assess the preparations for their return, including the rebuilding of schools and hospitals and the demining of roads.