The African Union has insisted its peacekeeping mission in Sudan's Darfur region will end in three months' time.
The 7,000-strong AU force is hampered by lack of funds
The peacekeepers will leave by the end of September even if there is no agreement on replacing it with a United Nations force, an AU meeting agreed.
Sudan is vehemently against this move, but UN boss Kofi Annan hopes to change their mind at this weekend's AU summit.
South Africa's foreign minister said the AU did not have the money to continue even if it wanted to.
But the UN head of peacekeeping, Jean-Marie Guehenno, says the UN is committed to bolstering the AU mission.
The 7,000-strong AU force in Darfur operates with the approval of the Sudanese government, but has been hampered by a lack of funding and resources.
Conflict in Darfur between rebels and pro-government forces has killed at least 200,000 people in three years it is estimated.
"Whatever happens our mandate ends on 30 September unless there are new developments in the discussions between the Sudan and the UN," said South Africa's Foreign Minister Nkosazana Dlaminini Zuma, who chaired an AU Peace and Security Council meeting on the subject.
"For us that mandate should end and the UN should be the one who takes over."
The meeting took place ahead of an AU heads of state summit in The Gambia.
In a separate move, the head of UN peacekeeping Jean-Marie Guehenno has said the UN will strengthen its support for the AU Darfur mission. But, he gave no details.
"We believe that the United Nations can help the African mission," he told reporters at the UN after returning from an assessment mission in Sudan.
"We did not get any objection from the government of Sudan so we are going to work in earnest on that."
If Sudan gave its consent, he said, a UN force the size of a division - thought to be about 17,000 troops - should be deployed in January next year.
A recent AU-brokered peace deal has failed to end the violence in Darfur, where more than 2m people have fled their homes.