Neither Rwandan President Paul Kagame nor his Congolese counterpart Felix Tshisekedi attended Friday's AU meeting.
A Congolese government source told AFP Tshisekedi would not attend the summit over the weekend either, saying: "He must closely follow the situation on the ground in DRC."
Tshisekedi, speaking at the Munich Security Conference on Friday, urged nations to "blacklist" Rwanda, condemning Kigali's "expansionist ambitions".
Having captured the key provincial capital of Goma last month, the Rwandan-backed armed group M23 pushed south.
Rwanda has not admitted backing M23 but has accused extremist Hutu groups in DR Congo of threatening its security.
Weak talks
As the AU summit opens, observers have branded the African Union ineffective in the DRC crisis.
"The AU has no power in this conflict and is playing the spectator," Thierry Vircoulon, of the French International Relations Institute (IFRI), told AFP.
International Crisis Group's Great Lakes project director Richard Moncrieff was also pessimistic about what the AU could achieve.
"Kagame has clearly calculated that his best approach is to push forward, and he does have some support," he told AFP.
"Some African leaders have trouble defending Congo because they don't defend themselves."
M23 rebels in Bukavu
M23 took a vital airport before marching virtually unchecked into another key city, Bukavu, on Friday, security and humanitarian sources said.
"I confirm that we entered Bukavu this evening, and tomorrow, we will continue with the operation to clean up the city," Corneille Nangaa, leader of the Congo River Alliance, which includes the M23, told Reuters.
Residents also reported seeing the militants in the streets of a northern district.
The Congolese army did not immediately respond to a request for comment.
Rwanda-backed M23 rebels pledge to 'march all the way to Kinshasa'
Earlier on Friday, the Congolese army confirmed that M23 fighters had taken control of Kavumu airport, north of Bukavu, and that Congolese troops had pulled back with their equipment.
The rebels have been trying to push south towards Bukavu since they seized Goma, in North Kivu, the largest city in eastern Congo, at the end of last month.
Any escalation in fighting could worsen the humanitarian situation, with the UN saying on Thursday that there had been an influx of displaced people moving towards the city, where nearly 1.3 million people already reside.
The capture of Bukavu, the capital of South Kivu province, would represent an unprecedented expansion of territory under the M23's control since the latest insurgency started in 2022, and deal a further blow to Kinshasa's authority in the east.
(with newswires)
