"There is a lot of banditry ... The area is lawless and they (gunmen) are attacking everyone," Jean Baptiste Natama, a senior AU protocol officer told Reuters.
Natama said one person was lightly injured on Thursday when unidentified gunmen attacked a patrol near Nyala, the capital of South Darfur state.
The AU official said the incident was the first of its kind in several weeks. But an aid official, who did not want to be named, said incidents of gunmen shooting at convoys and stealing vehicles and aid had increased in recent months in Darfur.
"The situation got worse from around April, May, June ... They (gunmen) are taking vehicles. We have lost trucks and aid commodities," said the official who would not say how much material had been lost.
Two main groups launched a rebellion in early 2003 accusing the government of neglect and arming Arab militias to burn and loot villages.
Fighting between Arab militias and rebels has decreased in recent months but tens of thousands of people have been killed and around 2 million people displaced by the violence remain in refugee camps dependant upon aid for survival.
The AU official said the African body was conducting investigations into the recent attacks but added it was not yet clear if the gunmen acted alone or had links to the other armed groups in the area.
The rebels and the government have said they will begin a sixth round of peace talks in Abuja on Sept. 15.