Extensive war crimes are being committed by both sides in the conflict that has been raging in Sudan since April, Amnesty International said on Thursday.

The Britain-based human rights group said in a report that the crimes committed by the warring parties, led by two feuding generals, included sexual violence against girls as young as 12 and the indiscriminate targeting of civilians.

Since April 15, regular army chief Abdel Fattah al-Burhan has been locked in a war with his former deputy, the paramilitary Rapid Support Forces (RSF) commander Mohamed Hamdan Daglo.

"Civilians throughout Sudan are suffering unimaginable horror every single day as the Rapid Support Forces and Sudanese Armed Forces recklessly vie for control of territory," said Amnesty secretary general Agnes Callamard.

"The RSF and SAF, as well as their affiliated armed groups, must end their targeting of civilians and guarantee safe passage for those seeking safety," she added.

Burhan came to power, with Daglo as his number two, in an October 2021 coup that derailed a fragile transition to civilian rule after the military's ouster of long-ruling autocrat Omar al-Bashir in April 2019 following a popular uprising.

But the two men then fell out in a bitter feud.

The fighting -- concentrated in Khartoum and the western region of Darfur -- has killed more than 3,900 people, according to the NGO ACLED and displaced more than 3.3 million, according to the UN.

"Extensive war crimes are being committed in Sudan as the conflict... ravages the country," Amnesty said, adding there were "mass civilian casualties in both deliberate and indiscriminate attacks by the warring parties".

It said men, women and children have been caught in the crossfire as both sides launch frequent attacks in densely populated residential neighbourhoods, often using explosive weapons with wide area effects.

Amnesty said scores of women and girls, some as young as 12, have been subjected to sexual violence, including rape, with some held for days in conditions of sexual slavery.

In most of the cases documented by Amnesty International, survivors said the perpetrators were fighters of the RSF or its Arab militia allies.

For its report, Amnesty said it had interviewed more than 180 people, primarily in eastern Chad where refugees from Darfur have fled, or remotely via secure calls.

The group said it had put its allegations to the army and the RSF, who had both responded "claiming adherence to international law and accusing the other side of violations".