The slain man was Nigerian, the U.N. spokesman's office said. The United Nations says it is working with Sudan's government to investigate the attack on the peacekeeping team's site near Muhajeria in East Darfur State.
An estimated 18,000 displaced persons remain concentrated around the U.N. team sites near Muhajeria and Labado villages since April 6, when fighting broke out between Sudanese government forces and the Sudan Liberation Movement/Minni Minawi, the U.N. said.
Secretary-General Ban Ki-moon was "appalled and saddened" by the killing, his spokesman's office said.
Ban also said he was extremely concerned about Sudan's restrictions on the distribution of U.N. aid to afflicted civilians in the area.
The joint African Union-U.N. UNAMID peacekeeping force was established in 2007 to protect civilians in Darfur, but also contributes to security for those providing humanitarian aid, verifying agreements, political reconciliation efforts and promoting human rights.
It has about 16,500 troops and military observers and over 5,000 international police. More than 300,000 people have been killed in the Darfur conflict since rebels took up arms against the Sudanese government nearly 10 years ago, accusing it of discrimination and neglect.