Antonio Guterres, UN high commissioner for refugees, said: "The situation is one of total insecurity, where humanitarian workers are risking their lives.
"There are members of our team who can't leave the areas, where they are, unless it is by helicopter, to work in the camps because the roads are considered to be totally unsafe."
Guterres said there had been cases where security forces in the country belonging to the African Union had been kidnapped by armed groups in Sudan.
Internally displaced people
The AU, which had a 6 000-strong peacekeeping force in the western region of Darfur, carried out patrols of camps for internally displaced people in Sudan.
Last month, militiamen entered a camp in Darfur and fired on civilians, killing two children.
War broke out in Darfur in 2003 after rebel groups begun fighting what they said were the political and economic marginalisation of the region's black African tribes by the Arab-led regime in Khartoum.
As many as 300 000 people had died and more than two million fled their homes in what UN aid agencies had dubbed the world's worst humanitarian crisis with reports of rapes, extra judicial killings and other atrocities rampant.