A Sudanese woman who fled El-Fasher in Darfur carries jerrycans of water at the Al-Afad camp for displaced people in the town of Al-Dabba, northern Sudan. AFP
The RSF seized El-Fasher at the end of October, the last major city in the vast western region of Darfur that had remained outside their control in its war against the Sudanese army.
The paramilitary group said on Monday it was declaring a unilateral three-month ceasefire.
Amnesty said it had collected testimonies from 28 survivors describing atrocities in El-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 El-Fasher who said she and her 14-year-old daughter were raped by RSF fighters as they fled the city.
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 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, General Abdel Fattah al-Burhan, the head of the Sudanese national Army, 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".
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