Authorities in Sudan's South Darfur province have expelled the head of the United Nation's humanitarian operations, Wael al-Haj Ibrahim.
More than two million civilians are estimated to live in refugee camps
He was accused of unspecified rule violations. The United Nations has deplored the expulsion.
A UN spokeswoman expressed concern about the potential ramifications of this decision.
The UN plays an important role in co-ordinating assistance to up to 1m internally displaced persons.
Mr Ibrahim, a Canadian national, head of the Office for the Coordination of Humanitarian Affairs (OCHA) in Nyala, South Darfur, "had been forced to leave" by provincial authorities, OCHA spokeswoman Stephanie Bunker said.
She said South Darfur's Governor, Ali Mahmood Mohammed, asked Mr Ibrahim in a letter to leave because "he was not complying with the rules of humanitarian action."
The OCHA said the expulsion violated the letter and spirit of the agreement it had with the authorities.
Mr Ibrahim's expulsion comes against a background of rising tension between humanitarian organisations and the authorities in Darfur, the BBC's Martin Plaut reports.
Late last month the UN witnessed troops and police rounding up refugees from a camp on the outskirts of Nyala.
Around 1,000 refugees were forced onto trucks at gunpoint and driven into the night.
According to a human rights organisation, the Aegis Trust, they were dumped on the outskirts of the city and have ended up living with friends and relatives.
Mr Ibrahim - as the head of the UN's humanitarian operations in Nyala - objected to the forced relocation.
But most have nowhere to go, and fear attacks if they leave the relative security of the camps, our correspondents says.
Few have decided to accept the offer of voluntary return and the Aegis trust is describing attempts to close the camps as a further ethnic cleansing of Darfur's African population, he adds.
Some 200,000 people are estimated to have died and more than two million displaced during the four-year war which has ravaged Darfur.