The United Nations is pushing to gain access to al-Fashir, the famine-stricken city in Darfur where witnesses have reported mass reprisals since a takeover by Sudan's Rapid Support Forces last month, UN aid chief Tom Fletcher said.

Fletcher told Reuters it would be a huge task to provide aid to the city, which would be treated as a "crime scene" for investigations following reports of systematic executions, detentions, and rapes.

Many of those thought to have remained in al-Fashir when the paramilitary RSF took control following a long siege are still unaccounted for. 

Safe passage was needed for humanitarians to enter the city and for survivors to leave, Fletcher said in an interview late on Tuesday from N'Djamena in Chad, following a visit to Darfur.    

Fletcher said talks with the RSF were "super delicate" but he hoped the UN would gain access in days or weeks, not months. “We will put in the hard work to get in," he said. 

ATROCITIES 'ON A HORRIFIC SCALE'

Al-Fashir's fall on October 26 has cemented the RSF's control of the Darfur region in its 2-1/2-year war with the Sudanese army. The city has been cut off from communications since the RSF offensive.

"There have been mass atrocities, mass executions, mass torture, sexual violence on a horrific scale," Fletcher said. "This is a city that has been under siege for so long, they’ll need food, water, medicine."

"There's a massive job ahead of us," he added.

The RSF says reports of atrocities have been exaggerated but that it was investigating cases of abuses by its soldiers. The International Criminal Court has said it is collecting evidence of alleged mass killings and rapes in al-Fashir.

Though more than 100 000 people are thought to have fled al-Fashir since the RSF takeover, only a fraction of those have reached the nearby town of Tawila, controlled by neutral forces.

Most of the rest are thought to be in inaccessible villages around al-Fashir.

PUSH FOR FULL ACCESS

Fletcher, who visited Tawila, where an estimated half a million displaced people were already sheltering, described the 350km journey from there to the border with Chad as "utterly perilous".   

Few people had the resources to get through an estimated 30-40 checkpoints along the route, "which is why it’s so urgent that we get the full authority to operate at scale inside Sudan -- inside Darfur, Tawila, and in al-Fashir," he said. 

Fletcher said aid deliveries would be contingent on the RSF providing safe passage for UN convoys as well as fleeing civilians, and providing accountability for fighters who have committed atrocities.

The UN aid chief also said he held talks with Sudan’s army chief General Abdel Fattah al-Burhan last week in Port Sudan for full access to the country. The Sudanese army has placed bureaucratic roadblocks to such access in the past.