In this May 24, 2022 photo, people walk on the road near Kibumba, north of Goma, Democratic Republic of Congo, as they flee fighting between Congolese forces and M23 rebels in North Kivu. AP
In a joint statement, AU chairman Macky Sall and AU Commission chair Moussa Faki Mahamat said they "express their extreme concern" at the failing security and appealed for calm and dialogue.
"They call on all the parties to establish an immediate ceasefire, respect international law, the safety and security of civilians," the statement said.
The M23 rebels seized more territory in the vast mineral-rich DRC on Saturday, prompting the UN peacekeeping mission to increase its "troop alert level" and boost support for the army.
The latest advance came as diplomatic relations between neighbours DR Congo and Rwanda worsened over support of the rebels.
The authorities in Kinshasa, who accuse Rwanda of supporting the M23 rebels, on Saturday announced the Rwandan ambassador would be expelled, a move Kigali said was regrettable.
The AU urged all the parties to engage "in a constructive dialogue" to ensure peace in the troubled region.
M23, a mostly Congolese Tutsi group, resumed fighting in late 2021 after lying dormant for years, accusing the government of having failed to honour an agreement over the demobilisation of its fighters.
It has since captured swathes of territory in North Kivu, including the key town of Bunagana on the Ugandan border in June.