UN humanitarian chief Jan Egeland has cut short his trip to Darfur and returned to Khartoum after the government blocked his access to camps housing Darfuris who have fled rape, murder and pillage.
Khartoum officials say security has improved in Darfur since a May peace deal signed by only one of three rebel negotiating factions.
But government security officials told Mr Egeland it was too dangerous for him to travel outside Darfur's state capitals.
Sudan's President Omar al-Bashir earlier said he welcomed the United Nations support for the African Union peacekeeping mission in Darfur, following reports of a deal.
The UN Secretary-General, Kofi Annan, said the Sudanese government had provisionally agreed to a joint UN-African Union peacekeeping force.
It follows intensive discussions in the Ethiopian capital, Addis Ababa.
He said the troops serving in Darfur would be African as far as possible, while the UN would provide the command and control structure.
Mr Annan said the size of the force had yet to be decided, but that he was optimistic a final agreement could be reached that would be acceptable to the international community.
Sudan has always rejected plans to replace the AU force with a larger, stronger UN mission. It remains unclear who would command the new force.
More than 200,000 people have died in three years of conflict in Darfur.
