U.N. staff visited the area and in March wrote an internal preliminary report on the situation, which Reuters has read.
"Girls and women are arrested wherever they are, without the necessary needs, detained and then separated from their children and husbands, subjected to inhuman and degrading treatment, sometimes raped," the report said.
The U.N. report did not say how many cases of abuse there were. But Victor Mikobi, a doctor who specializes in treating victims of sexual violence at a health center in Kamako, said local clinics had recorded 122 cases of rape this year, unprecedented levels for the town, he said.
The report, which would have to be checked by various partner organizations before any possible publication, did not explicitly identify the perpetrators. A doctor working in the area blamed civilians in DRC and Angolan security forces.
A spokesperson for Angola's migration authority, Simão Milagres, said there had been an increase in expulsions in the past few weeks but denied that rapes and other abuses had occurred.
"That's not true," he said. "I can guarantee that there isn't an institutional attitude promoting violence against migrants."
A DRC immigration official who spoke to Reuters on condition of anonymitys said that in meetings officials had talked about dozens of rapes on both sides of the border.
The governor of Kasai region in southern Congo, Dieudonne Pieme Tutokot, said he was aware of instances of rape and had opened an investigation.
Kamako has become an "open-air migrant camp", the head of the IOM's Congo mission, Fabien Sambussy, told Reuters.
Abbé Trudon Keshilemba, president of a group of civil society organisations in Kamako, said: "The DRC (people) end up occupying whole villages in Angola, and the Angolans feel that they will disappear."
The Angolan migration authority spokesperson, Milagres, said that the crackdown on illegal workers came as the country sought to promote legal migration through an online visa application process.
Mass deportations from Angola to Congo happen every few years. The largest, in 2018, led to the expulsion of 330,000 workers. Over the course of two months in 2010, the U.N. estimated that more than 650 people had suffered sexual violence during expulsions from Angola.
"We are witnessing this without being able to do anything due to a lack of resources," the DRC immigration officer said.