Rwanda-backed rebels have “occupied” a second major city in mineral-rich eastern Congo, Congo’s government said Sunday, as M23 rebels positioned themselves at the governor’s office in Bukavu and pledged to clean up after the “old regime.”

Associated Press journalists witnessed scores of residents cheering on the rebels after they entered Bukavu following a dayslong march from Goma, a city of 2 million people they seized last month.

The rebels saw little resistance from government forces against the unprecedented expansion of their reach after their years of fighting. Congo’s government vowed to restore order in Bukavu, a city of 1.3 million people, but there was no sign of soldiers. Many were seen fleeing on Saturday alongside thousands of civilians.

The M23 are the most prominent of more than 100 armed groups vying for control of eastern Congo’s trillions of dollars in mineral wealth that’s critical for much of the world’s technology. The rebels are supported by about 4,000 troops from neighboring Rwanda, according to the United Nations experts.

Congo’s President Felix Tshisekedi, whose government on Saturday asserted that Bukavu remained under its control, has warned of the risk of a regional expansion of the conflict.

Congo’s forces were being supported in Goma by troops from South Africa and in Bukavu by troops from Burundi. But Burundi’s president, Evariste Ndayishimiye, appeared to suggest on social media his country would not retaliate in the fighting.

The conflict was high on the African Union summit’s agenda in Ethiopia over the weekend, with U.N. Secretary-General António Guterres warning it risked spiraling into a regional conflagration.

Still, African leaders and the international community have been reluctant to take decisive action against M23 or Rwanda, which has one of Africa’s most powerful militaries. Most continue to call for a ceasefire and a dialogue between Congo and the rebels.

The Congo River Alliance, a coalition of rebel groups that includes the M23, has said it was committed to “defending” the people of Bukavu.

“We call on the population to remain in control of their city and not give in to panic,” alliance spokesperson Lawrence Kanyuka said in a statement Saturday.