The rare move on Thursday from the bloc creates fresh uncertainty into the post-election process, which was meant to usher in the vast country's first democratic transfer of power since independence in 1960 but has been mired in controversy.
"The Heads of State and Government attending the meeting concluded that there were serious doubts on the conformity of the provisional results, as proclaimed by the National Independent Electoral Commission, with the votes cast," the AU said in a statement after a meeting at its headquarters in Ethiopia's capital, Addis Ababa.
As a result, it has "called for the suspension of the proclamation of the final results of the elections".
The AU also agreed to urgently send "a high-level delegation" to the DRC's capital, Kinshasa, in an effort to find a way out of the political crisis.
Provisional results controversy
The electoral commission last week declared opposition leader Felix Tshisekedi the winner of the long-delayed December 30 vote with 38.57 percent of the tally against chief rival Martin Fayulu's 34.8 percent.
Fayulu, who is challenging the provisional results in court, said it was an "electoral coup" forged in backroom dealings between Tshisekedi and outgoing President Joseph Kabila, who has been in power since 2001.
The country's constitutional court is due to rule on the legal action later this month.
"Even if the situation on the ground has been fortunately calm so far, it obviously remains a cause for concern," AU chairperson Moussa Faki Mahamat told African leaders including from South Africa, Zambia and the Republic of Congo gathered to discuss the vote dispute.
Fayulu took to Twitter to thank "the African Union for its ongoing efforts in favour of truth and justice in the DRC", adding "we reiterate our call for a recount".
He has previously said he is not confident he will win before the nine-judge Constitutional Court, which he considers friendly to Kabila.
A Western diplomat, speaking on condition of anonymity, said the AU's move was unprecedented. "I cannot remember another instance where the AU called for a suspension of certification of results," the diplomat was quoted as saying by Reuters news agency.
|INSIDE STORY: An 'electoral coup' in the Democratic Republic of Congo? (25:15)|
Several international media outlets reported on Tuesday that leaked voting data showed that Fayulu had definitely won at the polls on December 30.
Britain's Financial Times, as well as France's RFI and TV5 Monde, said they were leaked full voting data – which had not yet been released – and that analysis showed Tshisekedi actually lost.
The United States on Wednesday kept up the pressure on Kinshasa, vowing to "hold accountable" anyone who undermines democratic processes.
Domestic election monitors noted a range of voting irregularities, and the influential Catholic Church said official results are inconsistent with its own tallies. Three diplomats briefed on the Church's findings said they show that Fayulu won a clear victory, according to Reuters.
In a report on Thursday, domestic election observer mission SYMOCEL called on the national electoral commission to publish results for each of the more than 50,000 polling stations. SYMOCEL also said that the commission had relied on results taken from voting machines with USB sticks rather than hand-counted tallies in legislative and provincial assembly elections held the same day, in violation of electoral law.
SADC backs off earlier recount call
Earlier this week, the Southern African Development Community (SADC), a bloc that includes Angola and South Africa, called for a recount of the vote and a unity government in DRC.
But in a communique issued on Thursday, SADC made no mention of those demands, instead calling on Congolese politicians to "address any electoral grievances in line with the Democratic Republic of Congo's Constitution and relevant electoral laws".
It also asked the international community to respect the DRC's "sovereignty" and "territorial integrity".
The vote dispute has raised fears that the country's political crisis, which erupted two years ago when Kabila refused to step down at the end of his constitutional term in office, could worsen.
The vast and chronically unstable country became a battlefield for two regional wars in 1996-97 and 1998-2003, and the last two presidential elections, in 2006 and 2011, were marked by bloody clashes.
The DRC is the world's leading miner of cobalt, a mineral used in electric car batteries and mobile phones, and Africa's biggest copper producer. It also mines gold and diamonds.
SOURCE: Al Jazeera and news agencies