Rights group Amnesty International on Tuesday accused Sudanese paramilitaries of committing war crimes in the Darfur town of Al Fasher.
The war pits the Sudanese army of General Abdel Fattah al-Burhan against the RSF headed by his former right-hand man, Mohamed Hamdan Dagalo.
The RSF seized Al Fasher at the end of October, the last major city in the vast western region of Darfur that had remained outside their control.
The militant group said on Monday it was declaring a unilateral three-month ceasefire.
Amnesty said it had collected testimonies from 28 survivors describing atrocities in Al Fasher ranging from the summary execution of unarmed men to the rape of girls and women.
"This persistent, widespread violence against civilians constitutes war crimes and may also constitute other crimes under international law," Amnesty chief Agnes Callamard said in a report published on Tuesday.
"All those responsible must be held accountable for their actions."
Tuesday's Amnesty report cites one survivor from Al Fasher who said she and her 14-year-old daughter were raped by RSF militants as they fled the city.
“One of them forced me to go with them, cut my Jalabiya [a traditional robe], and raped me. When they left, my 14-year-old daughter came to me. I found that her clothes had blood and were cut into pieces. Her hair at the back of her head was full of dust,” the survivor was quoted as saying.
She told Amnesty International that her daughter remained silent for the next few hours until she saw her mother crying: “She came to me and said, ‘Mum, they raped me too, but do not tell anyone.’
Her daughter became very ill when they reached the refugee town of Tawila, and died in a clinic there, the woman told the rights group.
Another 34-year-old survivor cited in the report said RSF forces killed people "like flies" and that none of those he saw killed was an armed soldier.
"As the conflict continues, the survivors' stories provide further proof of the failure of the international community in Sudan," Callamard said.
Mediation has so far failed to end the fighting as both sides try to secure military gains before talks are held.
On Sunday, Burhan rejected a US truce proposal from the Quad group of mediators as the "worst yet" and unacceptable.
The Quad group comprises the United States, Saudi Arabia, Egypt and the United Arab Emirates.
Later on Monday, the RSF declared a unilateral three-month ceasefire "in response to international efforts, including the initiative of US President Donald Trump and the Quad mediators".
