NAIROBI — Sudanese paramilitaries and allied militias have seized control of cities in the western region of Darfur from the government army, with mass killings reported in one regional capital and at a camp for displaced families, eyewitnesses said.

The capture of the cities, previously divided between the militias and the army, is the most significant military breakthrough by the Rapid Support Forces since the war began seven months ago, and threatens to usher in a new chapter of violence by drawing in forces that have previously kept aloof from the fighting.

Three of the five regional capitals have fallen in quick succession over the past two weeks: Nyala, Geneina and Zalingei. Major military bases have also been captured or deserted, with soldiers fleeing across the border into Chad.

Ahmed Sharif, 31, said Wednesday he had personally collected 102 bodies and laid tents over them after an attack over the weekend on Ardamata, a satellite settlement of Geneina that has an army base and a large camp for displaced families. The road to the border was strewn with dozens more bodies, he said, and leaders from the camp who had fled to Chad had collected the names of hundreds more people reported dead by family members and witnesses.

The civil war in the nation of 50 million people erupted April 15 between the RSF, headed by Gen. Mohamed Hamdan Dagalo, universally known as Hemedti, and the military, led by Gen. Abdel Fattah al-Burhan. The two men had cooperated to overthrow a government led by a civilian prime minister but then turned on each other.

The military has repeatedly bombed civilian neighborhoods, and the RSF, which is allied with several ethnically Arab militias, has been blamed for multiple attacks on hospitals and mass killings, as well as ethnically motivated attacks in the western region of Darfur. So far 6 million people in Sudan have fled their homes and half of the population needs urgent aid.

In Geneina’s Ardamata, the attack began Saturday morning from four directions, Sharif said, and after the paramilitary fighters took over the army base, they carried out killings and arrests in the city and camp.

“On Sunday, many four-wheel-drive vehicles came and entered the camp, and the killing continued until the evening,” he said. “After midday, they attacked us with motorcycles because cars cannot drive in those narrow streets. I hid in one of the houses, and I was hearing abuse and killing from nearby people. They said to people, ‘O slaves,’ and described us as allies of the army.” The ethnically Arab militias often use the term as an insult against ethnically African groups.

Another witness, Kaltoum Ahmed, 43, said she had seen militiamen kill around 30 people in the Kubra neighborhood near the Geneina army base after pulling them out of their homes.

A third witness, a 34-year-old who spoke on the condition of anonymity for his own safety, said he knew 10 people who were killed over the weekend. He said two of his relatives had tried to hide in a chicken house, but the militiamen had killed them there and he had seen the bodies. He said the Kubra neighborhood and Ardamata camp had been the site of large-scale killings. The United Nations and medical charity Doctors Without Borders also raised the alarm over the reports of atrocities.

“We have received reports of assassinations, massacres of families and civilians. We have heard about detentions and forced disappearances … of large numbers of people, including vulnerable internally displaced people, who had been sheltering around the army base in Ardamata. The RSF, as the de facto authority that now controls the region, are responsible for the safety of civilians,” said Toby Harward, the U.N. deputy humanitarian coordinator for Sudan.

Stephanie Hoffmann, a coordinator with Doctors Without Borders in Adré, a Chadian city on the border with Sudan, said more than 7,000 people had crossed the border in the first three days of November, more than the entire previous month. One man reported he had fled with 16 people — all of them had been killed on the road except for him. He was shot several times but survived by playing dead, the aid agency said.

The RSF didn’t immediately respond to a request for comment on the killings.

In the regional capital of Nyala in the southern part of Darfur, a witness said there were three days of violence and looting after the town fell on Oct. 26. He spoke on the condition of anonymity for fear of reprisal.

“The Rapid Support Forces told us that their forces do not have salaries from the state, so we must allow them to loot for three days,” he said. RSF fighters had set up checkpoints taking 500 Sudanese pounds (around a dollar) from those entering or exiting the market, he added. Police had abandoned their posts and their stations had been taken over by RSF fighters.

Forces affiliated with the RSF have surrounded another regional capital, Fashir, for the past five days. Fashir is home to tens of thousands of displaced civilians as well as heavily armed militias from the Zaghawa, an ethnically African group of people who spent nearly two decades fighting the central government. The Zaghawa have issued a statement saying they will fight if attacked.

The region’s governor in a Facebook post urged the warring parties to allow civilians to flee.

The threat of a clash between the Zaghawa and the RSF has raised fears that parts of Darfur could be plunged back into the civil war that devastated the area two decades ago, when Arab militias known as the Janjaweed — meaning “devils on horseback” — burned villages, killed civilians en masse and used mass rape as a weapon of war.

Later, many Janjaweed units were incorporated into the RSF while some of the former rebels received positions in government as part of a peace deal.

“We’ve already seen large-scale atrocities in Darfur at the hands of the RSF and affiliated militias,” said Alan Boswell, the Horn of Africa director for the International Crisis Group. “A battle for Fashir could be the tipping point that pushes parts of Darfur back into the kind of violence we saw 20 years ago.”

The violence follows months of bloody skirmishes in Sudan. The military controls most of the agricultural lands to the east, and the oil terminal in Port Sudan. The RSF controls the gold fields of the west, and the porous desert borders leading to refugee camps in Chad and the arms bazaars of Libya and the Central African Republic. It is also slowly extending its control of the oil pipeline that snakes from South Sudan to the sea.