The pan-African organisation, which aims to promote unity and democracy, says it has “serious doubts” about provisional results released last week.
Those figures gave opposition candidate Felix Tshisekedi victory but an opponent of the current administration, Martin Fayulu, insists he won.
The final results are due on Friday.
Questions have been raised about the accuracy of the results amid accusations that Mr Tshisekedi is planning a power-sharing deal with outgoing President Joseph Kabila.
A number of AU heads of state and government met in the Ethiopian capital, Addis Ababa, on Thursday and released a statement about the deeply disputed 30 December vote.
“There were serious doubts on the conformity of the provisional results, as proclaimed by the National Independent Electoral Commission, with the votes cast,” it read.