GOMA, Congo >> 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.

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

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,” according to 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.

“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.

Hundreds of thousands of Goma residents were trying to get away from the fighting, with some retreating into Congo’s interior to seek protection, and others crossing into nearby Rwanda.

Rwandan border patrol agents searched the belongings of those crossing the border in waves, with suitcases and children in tow. The displaced, looking exhausted but relieved, were then taken care of by the Rwandan Red Cross.

Meanwhile, rebels took control of Goma’s airport, said Dujarric, the U.N. spokesperson, warning of risks of a breakdown of law and order in the city, “given the proliferation of weapons.” Clashes with rebels continued in some parts of Goma on Wednesday afternoon, including the central neighborhoods of Katoyi and Kahembe. Other parts of the city were calm, with less fighting, a day after thousands of fleeing people hunkered down by roadsides as missiles flew overhead, and injured streamed into overwhelmed hospitals.

Tshisekedi was due to address the country later on Wednesday in first public remarks since rebels took Goma.

The chaotic situation has its roots in ethnic conflict: M23 says it is defending ethnic Tutsis in Congo. Rwanda has claimed the Tutsis are being persecuted by Hutus and former militias responsible for the 1994 genocide of 800,000 Tutsis and others in Rwanda. Many Hutus fled into Congo after the genocide.

Analysts say the real fight is for control over the Congo’s vast mineral deposits, estimated to be worth $24 trillion, and critical to much of the world’s technology. M23 rebels appear to be preparing to stay in Congo for good, and have told The Associated Press of their plans to set up an administration and return displaced people to their homes.