More than half of the 635 conflict-related detainees U.N. investigators interviewed said they had been tortured or treated badly, said the 139-page report, "Treatment of Conflict-Related Detainees in Afghan Custody: One Year On," by the U.N. Assistance Mission in Afghanistan (UNAMA).
The claims are exaggerated, the Afghan government said Sunday.
The report covered the year-long period of October 2011 and October 2012, and identified 14 methods of torture and ill-treatment electric shock, beatings, sexual abuse and threats of execution.
The report concentrated on detainees in facilities run by intelligence services and national and local police.
Incidents in police custody generally had gone up from 35% to 43% compared with the previous 12 month period, the report showed.
"UNAMA found a persistent lack of accountability for perpetrators of torture with few investigations and no prosecutions for those responsible," UNAMA Director of Human Rights Georgette Gagnon said.
"Without deterrents and disincentives to use torture, including a robust, independent, investigation process, criminal prosecutions and courts' consistent refusal to accept confessions gained through torture, Afghan officials have no incentive to stop torture."