Doctors Without Borders on Saturday said it feared an ongoing potentially fatal situation for "large numbers of people" in Sudan's El-Fasher, which has been captured by the paramilitary Rapid Support Forces.
Thousands of people have fled from El-Fasher, which fell to the RSF on October 26 after an 18-month siege.
Since then, testimonies of bloody violence targeting civilians have proliferated.
In a statement, Doctors Without Borders (MSF) denounced the "horrendous mass atrocities and killings, both indiscriminate and ethnically-targeted", that have occurred in and around El-Fasher this week.
"Large numbers of people remain in grave danger and are being prevented by the Rapid Support Forces and its allies from reaching safer areas, such as Tawila where we work," the NGO added.
But the numbers of people arriving to Tawila, a nearby region, "don't add up, while accounts of large-scale atrocities are mounting", according to MSF's head of emergencies Michel Olivier Lacharite.
"Where are all the missing people who have already survived months of famine and violence in El-Fasher?" he said.
"The most likely, albeit frightening, answer is that they are being killed, blocked, and hunted down when trying to flee."
Humanitarian organizations fear ethnically motivated atrocities similar to those committed in the early 2000s in Darfur by the Janjaweed militias, from which the RSF originated.
Several eyewitnesses told MSF that a group of 500 civilians, along with soldiers from the Sudanese Armed Forces and the army-allied Joint Forces, had attempted to flee on October 26, but most were killed or captured by the RSF and their allies.
Survivors reported that people were separated based on their gender, age or presumed ethnic identity, and that many are still being held for ransom. One survivor described "horrific scenes" where fighters crushed prisoners with their vehicles.
The war in Sudan has killed thousands of people, displaced millions more and triggered the world's worst humanitarian crisis, according to the United Nations.
The conflict erupted in April 2023 with a power struggle between two former allies: General Abdel Fattah al-Burhan, army chief and Sudan's de facto leader since the 2021 coup, and RSF chief General Mohamed Daglo.
