Nato countries risk turning a blind eye to torture by transferring prisoners taken in battle to Afghan prisons, a report from Amnesty International says.
It is difficult for Isaf troops to monitor prisoners' treatment
The report cites what it calls "consistent" incidences of torture and other abuse by Afghanistan's intelligence service, the NDS.
International law prohibits the transfer of prisoners if there is reason to suspect abuse or torture.
But Nato says Afghanistan has the legal responsibility for Afghan prisoners.
Amnesty wants transfers of those detained by Nato forces in Afghanistan to cease until proper safeguards are put in place.
Some members of the Nato-led International Security Assistance Force (Isaf) - including the UK, the Netherlands and Canada - have "memorandums of understanding" with the Afghan authorities, which are supposed to ensure that transferred detainees are treated according to international standards.
Responding to the report, the UK Ministry of Defence told the BBC: "The UK takes human rights obligations very seriously. Procedures are in place to ensure that any detainees transferred from British forces to Afghan government authorities are not mistreated or tortured."
A ministry spokesman said these safeguards include regular visits to transferred detainees, monitoring by both international and Afghan human rights groups, and British logistical help in allowing those rights groups to make visits to restive Helmand province.
"There is no evidence that any person detained by British forces and transferred to Afghan authorities has been tortured or mistreated," they went on to say.
But Amnesty says that proper monitoring of the agreements is impossible, partly because much of the country is inaccessible.
The report says that prisoners have been whipped, exposed to extreme cold and deprived of food.
And it concludes that all prisoner transfers should be suspended while efforts are made to improve Afghanistan's prisons and training is given to Afghan prison staff.
Only when the treatment of detainees in Afghanistan meets international standards should prisoner transfers resume, the report urges.
But Isaf is unwilling to change the current arrangement.
"Afghanistan is a sovereign country... which has the legal responsibility for detention of Afghans," said Isaf spokesman James Appathurai.
"It is not for Nato to create a parallel detention structure outside the law of the land," he added.
President Hamid Karzai's spokesman, Hamayun Hamidzadah, said the Afghan government condemned any kind of torture wherever it took place.
"If there are instances where torture has taken place, the Afghan government will investigate."
Amnesty said British authorities had confirmed that they were looking into the alleged torture of a transferred prisoner in September, and that Belgian officials had admitted losing track of a suspected suicide bomber after he had been transferred to the NDS.
The report specifically did not examine US troops' detention system in Afghanistan.