John Negroponte, deputy secretary of state, visited the AU compound on Saturday as part of a visit to Sudan aimed at persuading Khartoum to permit the deployment of UN troops in Darfur to bolster struggling AU forces.
Series of attacks
Saturday's incident was the third attack on AU troops in April, reinforcing fears that violence could undermine the world's biggest humanitarian effort.
Unidentified armed men attacked an AU peacekeeper patrol in the Darfur region of western Sudan on April 10, killing one and injuring two.
Five AU peacekeepers were also killed near Sudan's border with Chad after they also came under attack by assailants earlier in the month.
AU officials have said their 7,000-strong force is overstretched and under-equipped to police Darfur, a large region where violence has persisted despite a 2006 peace agreement between the government and one rebel faction.
Experts estimate about 200,000 people have been killed and 2.5 million have fled their homes since the Darfur conflict flared in 2003 when fighters took up arms against government forces, saying Khartoum had neglected the western region.
Some African countries with troops in the AU mission have threatened to withdraw their forces unless they are reinforced with more men and better equipment.
Sudanese officials working to finalise a deal on UN support for the AU mission in Darfur have recommended that Khartoum permit the use of attack helicopters by peacekeepers, the foreign ministry said.
The UN is nearing a deal with Khartoum to add 3,000 UN military personnel and equipment to the AU force under the so-called heavy support package but Sudan has so far objected to fielding six attack helicopters.
Sudan also has not agreed to the next stage of an AU-UN Darfur operation, which would involve 25,000 troops and police, saying it would amount to colonialism.