"We are deeply concerned," UN rights office spokesman Seif Magango said in a statement.

He pointed to an ultimatum issued earlier this week by the paramilitary Rapid Support Forces (RSF), at war with Sudan's regular army since April 2023.

In it, the RSF demanded that all army forces and allies leave El-Fasher by Wednesday afternoon, indicating it was prepared to carry out "an imminent offensive", said Magango.

The army in its response had "expressed its readiness to resist assault", he added.

"We renew our call on both parties to deescalate tensions around the city and to take urgent steps to ensure its civilian population is protected, consistent with their obligations under international law," he said.

Since April 2023, the war between army chief Abdel Fattah al-Burhan and his former deputy, RSF commander Mohamed Hamdan Daglo, has killed tens of thousands across the country.

It has also uprooted over 12 million people and created what the UN has called one of the worst humanitarian disasters in recent memory.

El-Fasher is home to some two million people who have been under RSF siege since May last year.

It has seen some of the worst fighting of the war as the army battles to keep its last foothold in the vast Darfur region of western Sudan.

"The people of El-Fasher have suffered so much already from many months of senseless violence and brutal violations and abuses, particularly in the course of the prolonged siege of their city" Magango said.