AU spokesman Aasane Ba said the group's Peace and Security Council on Thursday approved boosting the force from 2200 to more than 7700, including 5500 troops, 1600 civilian police and some 700 military observers.
AU Peace and Security Commissioner Said Djinnit said the enhanced force would be in place by the end of September. Kenya, Nigeria and Rwanda have pledged to contribute troops, he said.
"These extra troops will further promote a more secure environment and help build confidence as well as protecting civilians," Djinnit said.
The Darfur conflict broke out in February 2003 after rebels took up arms. The government is accused of responding by backing a scorched-earth counter-offensive by militias.
"These extra troops
will further promote a more secure environment and help build confidence as well as protecting civilians"
Said Djinnit, AU Peace and Security commissioner
War-induced hunger and disease has killed more than 180,000 people, according to UN estimates.
Djinnit said the Peace and Security Council did not discuss newly announced talks with Nato on possible logistical support.
He also said the council did not discuss changing the force's mandate to make it more robust.
The force currently has orders to monitor a shaky ceasefire signed last year, with only limited powers to protect civilians being attacked.
But an AU report has recommended that its troops should be allowed to intervene to protect civilians from violence.
UN special envoy to Sudan, Jan Pronk, meanwhile said a 12,000-strong peacekeeping force was needed in Darfur by early next year.
Pronk said the situation in Darfur had improved since last year, but said 500 people were still dying every month. He also said that serious violations of a ceasefire - most of them committed by rebels - were continuing.
The conflict in Darfur has been
raging since February 2003
"The AU presence has resulted in more stability where they are, but they have to be able to back their mediation with force," Pronk said.
African leaders have said they would also need more logistical support from rich countries to deploy a force of that size.
Sudan's ambassador to the AU, Abu Zaid al-Husain, said the group risks being seen by the Sudanese as an occupying force if its mandate is broadened to allow AU peacekeepers to step in and forcibly protect civilians.
"The protection of the civilians in Darfur should be left to the Sudanese civilian police," al-Husain said.