"An AU team was ambushed and one Malian officer was shot and another was slightly wounded," Nureddin Mezni, a spokesman for the African Mission in Sudan (AMIS), told AFP.
Mezni did not reveal the nationality of the other AU troop wounded in the incident, which took place Tuesday in the Shiariyah area of South Darfur state.
A press report said one civilian was also killed in the incident which broke out when AU troops were called in by villagers who had complained that rebels had stolen their cattle.
The AU has more than 2,000 monitors and military observers deployed in Darfur, alongside 112 civilian police, to oversee a widely-broken ceasefire agreed in neighbouring Chad nearly a year ago.
But in a vast arid region the size of France with very few roads, the force has struggled to respond promptly to allegations of violations by both sides.
According to a British parliamentary report released on Wednesday estimated at 300,000 the number of people of have died over of the past two years in Darfur.
The western Sudanese region has been devastated by the combined effect of a dire humanitarian crisis and a civil war between government forces and rebel groups complaining of marginalisation.