The Sudanese army and armed opposition groups in the region blame each other.
The UN statement said most civilians had fled after the September 29 attack, but a few had returned to search for food and water.
It was unclear if anyone had been killed or injured in the destruction of the town, but the UN said the market area had been looted.
An Associated Press reporter saw Haskanita intact last weekend just as the army was taking control following the suspected rebel attack.
A UN official who had just inspected the North Darfur town said on Sunday that more than 15,000 civilians were fleeing the area.
The official, who spoke on condition of anonymity, said the town had been destroyed by the Sudanese army and Janjawid fighters.
Several international observers, including aid workers and UN officials, disputed claims by local rebel chiefs that about 100 civilians had been killed in the destruction of Haskanita.
The Sudanese military had no immediate comment.
The AU said it could not comment since it had evacuated the area around Haskanita last Sunday.
The attack on the peacekeepers’ camp was the bloodiest against the undermanned and ill-equipped AU mission and threw into peril peace talks between the Sudanese government and rebels set for this month in Tripoli, Libya.
At the time of the attack on the AU peacekeepers, Sudanese forces were launching raids against rebel fighters in the region.
Some rebels have said the attack on the AU peacekeepers may have happened because some rebel groups suspected the AU of collaboration with Sudanese forces, something the AU denies.