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An air raid on Sudan's war-ravaged Darfur region killed at least 40 civilians Wednesday, according to a medical source, as the head of the UN mission to the country resigned.

Volker Perthes, who has been "persona non grata" by Sudanese authorities since June, warned the United Nations Security Council again, in his final briefing before leaving the post, that Sudan's war risks further deterioration.

"Forty civilians have been killed in an air strike that hit two markets and a number of the city's neighbourhoods," the medical source told AFP from a hospital in Nyala, the capital of South Darfur. The source asked for anonymity out of security concerns.

Since battles began on 15 April between the regular army, led by General Abdel Fattah al-Burhan, and the paramilitary Rapid Support Forces (RSF) commanded by Burhan's former deputy Mohamed Hamdan Dagalo, nearly 7,500 people have been killed, according to a conservative estimate from the Armed Conflict Location & Event Data Project.

More than five million people have been uprooted, including one million who fled across borders, according to UN figures.

Intensified fighting

Witnesses in Nyala had reported earlier on Wednesday air strikes falling on two markets and causing civilian casualties in Sudan's second-biggest city.

The western region of DarfurÌý– the size of France and home to a quarter of Sudan's populationÌý– had already seen some of the war's worst unrest before violence intensified last month.

Over 10 days in August, more than 50,000 people fled the city, according to the UN.

In early September, those who remained looked up to see a new escalation: air force fighter jetsÌý– whose strikes have been largely limited to the capital KhartoumÌý– flying overhead.

Their bombs struck both RSF bases and the residential neighbourhoods they inhabit, witnesses told AFP.

The army maintains control of the skies, and has been accused of repeated indiscriminate shelling of residential areas where paramilitaries have embedded themselves, including by evicting families and taking over homes.

Positioning themselves in civilian occupied neighbourhoods and buildings is "a potential violation of the Geneva Conventions," the US-supported Sudan Conflict Observatory has said.

It added that the Sudanese Armed Forces "would still be required to ensure that civilian harm is minimised regardless of whether a target has been made a legitimate military target."

Wednesday's attack came a day after a medical source reported 17 civilians killed in Khartoum's sister city of Omdurman. Witnesses described that attack as RSF shelling.

On Sunday, at least 51 people were killed and dozens wounded in air strikes on southern Khartoum, according to UN human rights chief Volker Turk.

The medical charity Doctors Without Borders (MSF) called it "the deadliest weekend witnessed by our teams in Khartoum since the beginning of the conflict, five months ago."

Call for accountability

In the war's early months, diplomatic efforts repeatedly failed to establish a sustained ceasefire and were instead "often used for repositioning and resupply" by both parties, Perthes told the Security Council.

Perthes has been repeatedly accused by Burhan and his allies of bias towards the RSF.

He has been persona non grata since he denounced possible "crimes against humanity" in Darfur.

The government repeatedly called for Perthes's dismissal, while UN Secretary-General Antonio Guterres reiterated his support for the envoy.

On Wednesday Guterres accepted the resignation of Perthes, saying "he has very strong reasons."

A UN team remains in Port Sudan, a coastal city spared the fighting.

"I am grateful to the Secretary-General for that opportunity and for his confidence in me, but I have asked him to relieve me of this duty," Perthes said, warning that the conflict "could be morphing into a full-scale civil war."

He added that the warring parties "cannot operate with impunity, and there will be accountability for the crimes committed."

While the deadly air strikes fell on Darfur Wednesday, witnesses in the capital also reported "columns of smoke rising" in central Khartoum as armed forces fighter jets "targeted RSF bases" in that area.

Until late last month, Burhan had been holed up in army headquarters, under siege by the RSF.

From his new base in Port Sudan, he has since visited Egypt, South Sudan, Qatar and Eritrea in what analysts say is a diplomatic push to burnish his credentials in the event of negotiations to end the war.

Burhan arrived in Turkey on Wednesday for his fifth trip abroad since late August, vying for legitimacy in his power struggle with Dagalo.

Burhan has been de facto head of state since he led a 2021 coup in collaboration with the RSF's Dagalo.

The army chief held talks with Turkish President Recep Tayyip Erdogan on "the course of bilateral relations and advancing the prospects for joint cooperation" between the two countries, according to Sudan's ruling Sovereign Council.