Rebels backed by Rwanda captured more towns in eastern Congo on Wednesday as they moved beyond the key city of Goma in an apparent attempt to expand their control in the conflict-battered region.

The rebels advanced toward the center of South Kivu province after taking several towns, including Kalungu, Kanyezire and Mukwinja, according to a local civil society leader and an aid worker in the area. They spoke on condition of anonymity because they were not authorized to talk to the media, AP reports.

UN spokesman Stephane Dujarric said the developments were concerning, and that the United Nations was taking measures to protect civilians and UN staff.

The advance also raised fears of a prolonged occupation by the rebels, who have said they plan to set up a new administration in Goma, a city of 2 million people.

“Left unchecked, the fighting could spread ... recalling the horrors of the late 1990s and early 2000s, when millions died,” the Crisis Group said, citing the last, multi-country war there.

Rwanda-backed rebels claim capture of Goma in eastern Congo

Unlike now, the rebels captured Goma only briefly during their first rebellion in 2012, withdrawing after a few days under international pressure on Rwanda.

Meanwhile, Congolese President Félix Tshisekedi was conspicuously absent from a virtual summit of a regional East African bloc that began late Wednesday, hosted by Kenya.

Tshisekedi was on an official visit to Angola, which had mediated a ceasefire between Congo and Rwanda last year, the Angolan presidency said in a statement. He was to discuss the situation in eastern Congo with Angola’s President João Lourenço.

Faced with the rebel offensive, soldiers fighting for Congo — many of them mercenaries from other nations — were laying down their weapons at the border with Rwanda. Congo’s army seemed to be in disarray, and those at the border appeared disorganized and ill-equipped for any major fighting.

A rebel spokesman said nearly 300 mercenaries have surrendered their arms and returned to their countries.

“Let the people of Goma be calm, peace is here,” said Willy Ngoma, a spokesman of the M23 group. Ngoma, who is on the U.N. Security Council’s sanctions list for crimes committed by the rebels.

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“It’s like you are fighting without command,” said Jean Marie Ndahambaza, one of the surrendering soldiers.

Armed groups have long vied for control of eastern Congo, which is rich in minerals critical to much of the world’s technology, and has been the scene of proxy battles between Congo and neighboring Rwanda, as well as other powers. Fighting reignited this week, and the rebels backed by Rwanda, known as M23, announced Monday that they had captured much of the provincial capital of Goma.