Since the Darfur town of al-Fashir was taken over by the RSF militia at the end of October, the situation has deteriorated rapidly and acutely, according to the UN chief.
Hundreds of thousands of civilians are trapped in this siege, says Guterres.
During an ongoing UN conference in Qatar, he appeals to the warring parties to sit down at the negotiating table and "put an end to the nightmare of violence - now".
Al-Fashir and the surrounding areas in North Darfur are an epicenter of suffering, hunger, violence and forced displacement.
The RSF (Rapid Support Forces) has been engaged in a bloody war with the Sudanese army since April 2023. The war broke out after divisions arose within the country's ruling military junta, which had seized power a couple of years earlier after the fall of long-time dictator Omar al-Bashir.
On one side stands General Abd al-Fattah al-Burhan, who commands the regular army. On the other is his former deputy Mohamed Hamdan Dagalo, also known as Hemedti, who leads the RSF.
Since RSF took al-Fashir, the city has been largely cut off from the outside world. But the reports that are actually making their way out are increasingly worrying, notes UN chief Guterres.
There are continuous reports of violations of international law and human rights, he says.
