African Union Commission Chairperson Mahmoud Ali Youssouf "expresses deep concern over the escalating violence and reported atrocities" in El-Fasher, said a statement on X, condemning "alleged war crimes and ethnically targeted killings of civilians".
He also called for an "immediate cessation of hostilities and the opening of humanitarian corridors to allow life-saving aid to reach affected populations".
Youssouf emphasised there could be "no military solution" to the crisis, urging all parties to engage in dialogue to "commit to a peaceful, inclusive political process".
Yale University's Humanitarian Research Lab said on Monday it had found "evidence consistent with Rapid Support Forces (RSF) conducting alleged mass killings after capturing El-Fasher, North Darfur, according to satellite imagery analysis".
Sudanese army allies the Joint Forces accused paramilitaries of having "executed more than 2,000 unarmed civilians" since taking control of the western city of El-Fasher on Sunday, as alarming reports of atrocities emerge.
The Rapid Support Forces (RSF) "committed heinous crimes against innocent civilians in the city of El-Fasher, where more than 2,000 unarmed citizens were executed and killed on October 26 and 27, most of them women, children and the elderly", the Joint Forces said in a statement on Tuesday.
Sudan's army admitted on Monday it had withdrawn from the strategic hub of El-Fasher, while the United Nations issued a stark warning over reports of "atrocities" by the paramilitary group now in control of the city.
Fears have mounted for the safety of tens of thousands of civilians in the city in western Darfur, trapped in an 18-month siege by the Rapid Support Forces (RSF), which has been fighting a brutal war with Sudan's army for more than two years.
"We have agreed to withdraw the army from El-Fasher to a safer location," the President of Sudan, General Abdel Fattah al-Burhan, said in a speech broadcast on national television, after the RSF announced victory on Sunday.
Burhan also vowed revenge and to fight "until this land is purified".
The capture of El-Fasher could mark a significant turning point in Sudan's war, which has killed tens of thousands and displaced nearly 12 million people since April 2023.
The city's capture gives the RSF control over all five state capitals in Darfur, consolidating its parallel administration in Nyala, the capital of South Darfur.
The army is now confined to the north, east and centre of Sudan and is excluded from a third of Sudanese territory, a development, experts say, which raises the possibility the country could face partition.
"This represents a terrible escalation in the conflict," UN Secretary-General Antonio Guterres said in answer to an AFP question, adding that "the level of suffering that we are witnessing in Sudan is unbearable".
UN rights chief Volker Turk spoke of a growing risk of "ethnically motivated violations and atrocities" in El-Fasher.
'Summary executions'
His office said it was "receiving multiple, alarming reports that the Rapid Support Forces are carrying out atrocities, including summary executions".
The UN Human Rights Office said the reports consisted of "summary execution of civilians trying to flee, with indications of ethnic motivations for killings", as well as videos showing "dozens of unarmed men being shot or lying dead, surrounded by RSF fighters who accuse them of being (Sudanese army) fighters".
A video released by local activists and authenticated by AFP shows a fighter known for executing civilians in RSF-controlled areas shooting a group of unarmed civilians sitting on the ground at point-blank range.
Footage shared by pro-democracy activists purportedly showed dozens of people lying dead on the ground alongside burned-out vehicles.
AFP was unable to contact civilians in the city, where the Sudanese Journalists' Syndicate says communications, including satellite networks, have been cut off by a media blackout.
The syndicate expressed "deep concern for the safety of journalists" in El-Fasher, adding that independent reporter Muammar Ibrahim has been detained by RSF forces since Sunday.
The UN Human Rights Office noted hundreds of people have reportedly been detained while trying to flee, including a journalist.
The head of the World Health Organisation (WHO) expressed outrage at the attack on the only hospital still partially operational in El-Fasher.
"According to reports, a nurse lost her life and three other health workers were injured" on Sunday, Tedros Adhanom Ghebreyesus wrote on X.
The army-aligned governor of Darfur called for the protection of civilians in El-Fasher. He demanded "an independent investigation into the violations and massacres carried out by the militia away from public view".
'Turning a blind eye'
According to the UN, more than one million people have fled the city since the start of the war and around 260,000 civilians, half of them children, remain trapped in El-Fasher without aid, where many have resorted to eating animal fodder.
The UN's migration agency said more than 26,000 people had fled the fighting in El-Fasher since Sunday, either seeking safety in the outskirts of the city or heading to Tawila, 70 kilometres (45 miles) to the west.
In Tawila, teams from Doctors Without Borders (MSF) said they were facing a massive influx of wounded "coming from El-Fasher to the overwhelmed town's hospital".
Since Sunday evening, 130 have been hospitalised, including 15 in critical condition, MSF said in a statement.
The RSF said they were conducting "comprehensive search and clean-up operations" to "eliminate the last pockets of terrorists and mercenaries".
The paramilitary group also claimed to have deployed teams to "protect civilians," and "secure the streets" by clearing away ordnance and debris.
The Emergency Lawyers, a group which has been documenting atrocities in Sudan, accused paramilitaries of "deliberately turning a blind eye to the actions of their members who carry out mass executions of civilians and prisoners", denouncing "systematic practices amounting to war crimes."
The UN last month voiced alarm over potential massacres targeting non-Arab communities in El-Fasher, similar to those reported after the RSF captured the nearby Zamzam camp in April.
Now well into its third year, the war has spiralled into what the United Nations describes as the world's largest displacement and hunger crisis.
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