More than 1,100 civilians, including journalists, lawyers and human rights activists, have been killed in West Darfur's capital of El Geneina since fighting began there in April, according to Sudanese human rights defenders.
Activists have reported widespread sexual violence, ethnic cleansing and bodies scattered in the streets of the city.
El Geneina has been under near-complete siege for over a week, as members of the Rapid Support Forces paramilitary and allied militias allegedly target ethnic African tribal leaders - most notably the emir of Dar Masallit area, Tariq Abdelrahman Bahreldin.
"Most parts of El Geneina are now besieged by these gunmen and the RSF," unnamed activists told Radio Dabanga on Monday, adding that snipers have been deployed in the city as well, while government forces and police are nowhere to be seen.
The West Darfur Doctors Union said on Monday that "what happened in Rwanda is now being repeated in El Geneina," in reference to the 1994 Rwandan genocide and the violence that precluded it.
Medical organisation Medecins Sans Frontieres (MSF) has described the city as currently "one of the worst places on earth".
"This is the harsh reality of the people who have been stranded there for weeks," MSF said, adding that they are struggling to provide medical care to civilians injured by the fighting.
Most residents have fled, facing attacks en route and multiple militia checkpoints.
The RSF even control some of the border crossings that residents from El Geneina have used to flee to bordering Chad.
"There is an emerging pattern of large-scale targeted attacks against civilians based on their ethnic identities, allegedly committed by Arab militias and some armed men" in RSF uniform, Volker Perthes, head of the UNITAMS mission to Sudan, said in a statement.
But some observers have become increasingly frustrated with the unwillingness of UN officials, including Perthes, to name the RSF as perpetrators of the horrific violence.
"Perthes couches his statement by referencing 'men in RSF uniform' - residents of the cities have identified RSF as being involved. Is there some legitimate reason to question whether RSF is involved, and if so, are they doing anything to mitigate civilian harm in these places?" asked African Affairs specialist Lauren Blanchard.
The RSF has said it is trying to counter looting, and has denied responsibility for the violence in Darfur.
The conflict has displaced more than 1.9 million people already, triggering a major humanitarian crisis that threatens to spill across a volatile region.
Fighting has been mostly concentrated in Khartoum, Omdurman, Kordofan and Darfur.