KHARTOUM, Oct. 27 (Xinhua) -- Dozens of civilians have been killed in El Fasher in western Sudan's Darfur region as the Rapid Support Forces (RSF) claimed to have seized the city, and the United Nations warned of a worsening humanitarian catastrophe.

The Sudan Doctors Network, a volunteer medical group, said the RSF carried out "ethnically based executions" and a "horrific massacre" in El Fasher, with early reports suggesting the death toll had reached dozens. The group accused RSF fighters of looting hospitals and pharmacies, leaving the sick and wounded without care.

Local activists said thousands of civilians trying to flee the city risked being killed or detained by RSF forces. The Coordination of Resistance Committees in El Fasher urged international intervention to create safe corridors for those trapped by the fighting.

The RSF on Sunday said it had taken full control of El Fasher, calling the city's fall "a pivotal moment" due to its "symbolic, strategic, and military significance."

El Fasher, the Sudanese army's last stronghold in the vast Darfur region, has been under intense siege by the RSF since last year.

Abdel Fattah al-Burhan, chairman of Sudan's Transitional Sovereign Council and commander of the Sudanese Armed Forces, said the Sudanese army has withdrawn from El Fasher. Meanwhile, Sudan's Ministry of Culture, Information and Tourism condemned "massacres, killings, torture, looting, and robbery against unarmed civilians" by the RSF in El Fasher and the nearby town of Bara.

The United Nations and aid agencies voiced alarm at the deteriorating situation. The International Organization for Migration said it was tracking new waves of displacement from El Fasher as families fled with little access to food, water, or shelter.

Residents remain "trapped and terrified-shelled, starving, and without access to healthcare or safety," the UN humanitarian office (OCHA) said, renewing calls for an immediate ceasefire "in El Fasher, in Darfur, and throughout Sudan."

UN Secretary-General Antonio Guterres on Monday warned of a "terrible escalation" of violence, while OCHA head Tom Fletcher on Sunday urged warring parties to allow safe passage for civilians.

In Bara, in the neighbouring Kordofan region, the Sudan Doctors Network said the RSF executed at least 47 unarmed civilians, including nine women, accusing them of links to the army. The group also reported looting, kidnappings, and other abuses.

Sudan's civil war, now in its third year, pits the Sudanese army against the RSF. The conflict has killed tens of thousands, displaced millions, and pushed much of the country to the brink of famine, despite repeated ceasefire calls from the United Nations and regional mediators.