At least 6,000 people were killed in three days, according to a report by the UN human rights office.
The offensive against al-Fashir - which was the regular army's last stronghold in Darfur - triggered "an intense wave of violence".
"The actual death toll during the week-long offensive is undoubtedly significantly higher," the UN office writes.
The UN has interviewed over 140 victims and witnesses. The accounts confirm previous reports of massacres, torture and sexual violence.
Sudan's civil war, which has raged since April 2023, has forced almost eleven million people to flee and pushed several regions into famine.
