(LONDON) — The U.S. is warning of a looming potential “large scale massacre” in El Fasher, a city in Sudan, as troops from the Rapid Support Forces paramilitary group and allied militias encircle the city in what the U.S. says are “indications of an imminent offensive.”

U.S. Ambassador to the U.N. Linda Thomas-Greenfield warned that El Fasher is “on the precipice of a large-scale massacre,” calling on the RSF to end its siege and swear off any attack on the city.

“A crisis of epic proportions is brewing in El Fasher,” said Thomas-Greenfield.

El Fasher — also known as Al-Fashir — is a city in North Darfur and one of Darfur’s largest cities. The besieged city is home to an estimated 800,000 people, according to the U.N., including many who have been displaced by the civil war that began more than a year ago. The city is also a key humanitarian hub for western Darfur, the vast area home to around a quarter of Sudan’s population.

At least 43 people, including women and children, have been killed since April 14, amid escalating violence between the Sudanese Armed Forces, or SAF, and the RSF and its allied militia as the paramilitary group began its push into the city. Multiple villages west of El Fasher have been razed to the ground by the RSF and allied militias, as the SAF conducted aerial bombardments in the region.

“Reports indicate that both parties have launched indiscriminate attacks using explosive weapons with wide-area effects, such as mortar shells and rockets fired from fighter jets, in residential districts,” said U.N. High Commissioner for Human Rights Volker Türk.

“Civilians are trapped in the city, the only one in Darfur still in the hands of the SAF, afraid of being killed should they attempt to flee,” Türk said. “This dire situation is compounded by a severe shortage of essential supplies as deliveries of commercial goods and humanitarian aid have been heavily constrained by the fighting, and delivery trucks are unable to freely transit through RSF-controlled territory.”

Writing on X, RSF Commander Mohamed Hamdan Daglo — known commonly as Hemedti — described the situation in North Darfur state as “critical,” saying the RSF is exercising its “legitimate right to self-defense in response to the aggression that we have faced since this war began.”

Fighting erupted between the Sudanese Armed Forces and the Rapid Support Forces paramilitary group last April following weeks of tensions linked to a plan for returning the country to civilian rule. The conflict has precipitated one of the world’s ‘worst humanitarian disasters in recent history’, displacing over eight million people.

As the conflict entered its second year, at least 14,000 people have been killed and almost 30,000 have been injured, according to the UN. local groups, which warn the true toll likely much higher.

Humanitarian organizations told ABC News that the situation in Zamzam camp, around 10 miles south of El Fasher, is “catastrophic.” About 30% of the children in the camp have been found to be suffering from malnutrition as war wounded pour in.

“In Zamzam camp, there is an acute disaster on a catastrophic scale,” said Claire Nicolet, head of Doctors Without Borders’ emergency response in Sudan. “The situation is critical and the level of suffering is immense, but despite this being known for nearly three months, nowhere near enough has been done to help those who are struggling to survive.”

The U.S. has called on all armed forces to immediately cease their attacks in El Fasher, saying, “An offensive against El Fasher city would subject civilians to extreme danger, including the hundreds of thousands of displaced persons who have taken refuge there.”

“The leaders of the SAF and RSF and their affiliated militias face a choice – escalate the violence and perpetuate the suffering of their people while risking the disintegration of their country, or cease attacks, allow unhindered humanitarian access, and prepare in good faith for negotiations to end this war and restore power to the people of Sudan,” Matthew Miller, a State Department spokesperson, said in a statement.

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