The Canadian military quietly stopped transferring detainees in Afghanistan to the control of Afghan authorities due to concern over torture allegations, according to a government letter released on Wednesday.

Canada stopped the transfers in November but did not disclose the change in policy until this week, according to Amnesty International and the British Columbia Civil Liberties Association, which released the letter.

The groups have sued the federal government over the transfer policy, and the letter from Canada's Department of Justice was sent to the groups as part of that case.

Justice officials said the transfers were stopped after receiving a report in November from a Canadian monitoring group with a credible allegation of mistreatment at an Afghan detention facility.

A secret government report released earlier in the week as part of the case said Canadian monitors in November found an electrical cable and rubber tube in a room where a prisoner said he had been beaten. The prisoner also had bruises.

The allegation was under investigation and "Canada will resume transferring detainees when it believes it can do so in accordance with its international legal obligations," the department said in the letter.

International conventions prohibit a country from handing over prisoners if there is a reason to suspect they will become victims of abuse.

Canada has 2,500 combat troops in the southern city of Kandahar, a region where the Taliban are concentrated.

The federal Conservative government had repeatedly denied there was evidence of torture, but political uproar over the issue prompted Canada and Afghanistan to sign an agreement allowing Canadian officials unfettered access to any prisoners handed over by Canada.

Prime Minister Stephen Harper's office said late on Wednesday that decisions on "operational matters" such as the transfer of prisoners were up to the Canadian military and it would not comment on them.

"Our military respects their international obligations," the prime minister's office said.

Amnesty International said the torture of detainees after the May agreement was "predictable and avoidable," and the rights groups vowed to continue their lawsuit against the government until transfers were stopped indefinitely.

"The government's decision (to stop transfers) amounts to a concession that the May 2007 monitoring agreement has failed to prevent torture by Afghan authorities," said Jason Gratl, president of the British Columbia Civil Liberties Association.