"The fighting must stop at once. Enough is enough," Volker Turk said in a statement.

El-Fasher is one of five state capitals in Sudan's western Darfur region and the only one not in the hands of the Rapid Support Forces (RSF), who have been battling Sudan's SAF regular army since April 2023.

Darfur, a region the size of France and home to around a quarter of Sudan's population, is deeply scarred by years of ethnic violence committed by the Janjaweed -- the militia from which the RSF emerged.

The RSF launched an offensive last weekend in El-Fasher, a city of some two million people, after a months-long siege.

"From bitter past experience, if El-Fasher falls, there is a high risk of ethnically-targeted violations and abuses, including summary executions and sexual violence, by the RSF and allied militia," Turk warned.

In particular, he highlighted concerns for residents of the Abu Shouk and Zamzam camps for internally displaced people.

"People in those camps are at grave risk of retaliatory attacks based on their tribal identity, real or perceived, as coming from the same communities as leaders of armed movements aligned with the SAF," the rights chief said.

The statement pointed to UN documentation of "horrific patterns of ethnically-targeted violations and abuses, specifically against the Masalit community", after the RSF seized control of El Geneina and Ardamata in West Darfur last year.

Amid the escalating battle for El-Fasher, Turk said his office had already documented "increasing incidents of civilians being killed as a result of shelling and airstrikes" by both sides.

The rights office said it had also documented cases of summary executions, sexual and gender-based violence and reported abductions of at least five women and several young men in El-Fasher.

Turk urged "the international community, including through the Security Council, to take necessary, effective action to protect civilians in Sudan, notably those groups at special risk of targeted violence, and to ensure respect for international law by all parties".