More than 500 people may have been tortured or starved to death and then buried in a secret mass grave north of Khartoum, according to evidence seen by the Guardian.

A visit to a base belonging to the paramilitary Rapid Support Forces (RSF) shortly after it was retaken by the Sudanese military found a previously unknown detention centre, with manacles hanging from doors, apparent punishment chambers and bloodstains on the floor. Accounts from people held at the detention centre describe being repeatedly tortured by their captors.

Nearby was a large burial site with at least 550 unmarked graves, many of them freshly dug and a number apparently containing multiple bodies.

The site is the biggest makeshift burial ground found in Sudan during its civil war and, if confirmed, would make this one of the worst war crimes of Sudan’s brutal conflict.