Afghan police and intelligence agents persist in torturing suspected insurgents through beatings, electric shocks and other means, despite foreign efforts to curb abuse, the United Nations says.

The UN issued a follow-up to a report on torture a year ago, as Kabul seeks full control over prisons and prisoners from NATO's International Security Assistance Force despite the misgivings of the US-led ISAF.

Other forms of torture included hanging suspects by the wrists from chains for long periods and threatening them with sexual violence, the UN mission in Afghanistan said in its 139-page analysis released late Sunday.

Many of those tortured to extract confessions were children under the age of 18, it said.

In October 2012, ISAF suspended the transfer of detainees to some Afghan facilities for a second time over the reports of torture.

"ISAF subsequently stopped transferring detainees to several Afghan facilities and implemented a process limiting transfer to a reduced number of Afghan facilities and increasing monitoring and accountability," the UN report said.

President Hamid Karzai has pressed hard for the transfer of prisoners to full Afghan control, saying it is an issue of sovereignty ahead of the withdrawal of NATO forces in 2014.

In November, Karzai accused the US of breaching an agreement to transfer more than 3,000 detainees at Bagram prison -- sometimes called the Guantanamo Bay of Afghanistan -- to Afghan control.

A spokesman for the president conceded there may be some cases of abuse, but denied that torture was government policy.

"The Afghan government is not implicated in crimes against detainees and torture and abuse of prisoners is certainly not our policy," spokesman Aimal Faizi said Monday.

"However, there may be certain cases of abuse and we have begun to investigate these cases mentioned in the UN report. We will take actions accordingly.

"While the Afghan government takes very seriously the allegations made in the UN report, we also question the motivations behind this report and the way it was conducted," he said.

The controversy over torture comes as Washington negotiates a security pact with Kabul covering relations after 2014, including the question of whether any US troops will remain behind to assist in the fight against Taliban insurgents.

Apart from the control of prisoners, the issue of immunity from prosecution in local courts for US troops is a major stumbling block in negotiations between the prickly allies.

The UN said that more than half of the 635 conflict-related detainees it interviewed experienced torture in facilities run by the police and the National Directorate for Security between October 2011 and October 2012.

The findings of the UN mission "are a cause for serious concern", said Jan Kubis, the UN special representative in Afghanistan.

"The government's attention and efforts to address these abusive practices are visible and encouraging, and have produced some positive results, but the system isn't robust enough to eliminate ill-treatment of detainees. Clearly more needs to be done to end and prevent torture."

The report said there was a persistent lack of accountability for torturers with few investigations and no prosecutions of those responsible.

The UN said the findings reinforced the urgent need for reforms in the judiciary, prosecution and law-enforcement sectors. Its recommendations included the creation of an independent national preventive mechanism on torture.