U.N. Secretary-General Antonio Guterres on Thursday urged an immediate halt to the escalating violence in Sudan following reports that paramilitary forces killed more than 460 people in a maternity hospital.

Mohammad Hamdan Daglo, the head of the Rapid Support Forces (RSF) paramilitaries, which recently seized the city of el-Fasher from army forces, has vowed the country would be unified by "peace or through war."

The capture of el-Fasher, the last army holdout in the vast western region of Darfur, comes after more than 18 months of brutal siege, sparking fears of a return to the ethnically targeted atrocities of 20 years ago.

Accusations of mass killings have mounted, with the World Health Organization (WHO) condemning reports that 460 people were killed at the Saudi Maternity Hospital, the last partially functional hospital in el-Fasher.

The WHO said the hospital was on Sunday "attacked for the fourth time in a month, killing one nurse and injuring three other health workers."