The chief of the AU commission, Alpha Konare, said that if such a move is not made in order to apply a peace agreement reached between the government and rebels in May then the security situation "could be very bad".
Konare, speaking after a meeting with the British prime minister, Tony Blair, said the switch in forces needed to be made before the rainy season starts in two months.
"The credibility of the agreement lies in making sure the undertakings are applied. We must lose no more time. If there is any doubt, everything comes into question."
Konare’s call came as senior UN diplomats began talks in Khartoum to try to persuade Sudan to agree to UN peacekeepers.
Change of heart
The government initially resisted such a proposal on the grounds the presence of UN troops would prompt attacks by Islamist militants but has relented since signing a peace deal with rebels on May 5.
It now says it wants to be consulted about the proposed UN force’s mandate in Darfur.
250,000 people have fled their
homes since January
The peace deal has not halted the violence in the region however, with dozens killed in clashes between rebels and government-armed Arab militias.
Meanwhile, two senior UN diplomats are in Khartoum to discuss the deployment of UN troops with senior members of the government.
Veteran negotiator Lakhdar Brahimi and the head of UN peacekeeping, Hedi Annabi, have met the deputy foreign minister Al-Samani al-Wasiyla and the AU mission head Baba Gana Kingibe and were expected to meet Omar Hassa al-Bashir on Thursday.
"We are hoping that we can work out an agreement with the government because ... this should not be done without the agreement of the government," said the UN deputy spokesman, Bahaa Elkoussy.
The 7,000-strong AU force has been trying to maintain a widely ignored truce in Darfur, but since the deal signed on May 5 Arab Janjaweed militias have grown bolder and attacked towns where the AU has bases.
More than 250,000 people have fled their homes since the beginning of the year because of the conflict. Frustrated Darfuris have begun to attack the AU force, killing an interpreter earlier this month.
"The credibility of the agreement lies in making sure the undertakings are applied"
Alpha Konare, AU commission
UN Secretary-General Kofi Annan has warned the Sudanese government that its restrictions on supplies and relief workers in Darfur is a violation of international humanitarian law.
He said in a report delivered to the UN Security Council on Monday that atrocities, including rape and pillaging, had swelling the population in camps, about 2.5 million.