There is a "reasonable basis" to believe that U.S. soldiers and intelligence agents committed the war crimes of torture and rape in Afghanistan, the chief prosecutor of the International Criminal Court says in a report.

It is the first time the prosecutor has used such strong language to describe U.S. conduct in Afghanistan, and it increases the likelihood of a formal war-crimes investigation against the United States. A decision will be made "imminently," the prosecutor said on Monday.

A formal war-crimes investigation against the United States would counter-balance the ICC's frequent prosecutions of African suspects. Three African countries have quit the court in recent weeks, saying it is biased against Africa. But an investigation of U.S. forces would provoke a furor in Washington. President-elect Donald Trump has often called for "interrogation techniques" such as waterboarding that are widely regarded as torture.

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In addition to the alleged U.S. crimes, there is also a "reasonable basis" to believe Afghan security forces and Taliban insurgents committed war crimes, according to the report released on Monday by the chief prosecutor, Fatou Bensouda, who has been conducting a preliminary review for several years.

She said the torture by the Afghan police and intelligence agents has been so widespread that "a state of total impunity persists." Even today, an estimated 35 per cent to 50 per cent of conflict-related detainees may be subjected to torture in Afghan detention facilities, she said.

Canadian activists and human-rights experts believe Canadian military conduct could also be investigated for possible war crimes if the ICC goes ahead with a full investigation, since Canadian troops allegedly transferred Afghan detainees to the Afghan security forces at a time when torture was common.

Foreign Affairs Minister Stéphane Dion, in an interview with The Globe and Mail last week, would not comment on the possibility of the ICC investigating Canada for war crimes, but said he had "confidence" in the ICC as an institution. He said Canada would work with the court.

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Most of the alleged U.S. war crimes were committed in the 2003 to 2004 period, in Afghanistan and in secret detention facilities operated by the Central Intelligence Agency, but some continued until 2014, the prosecutor's report said.

"These alleged crimes were not the abuses of a few isolated individuals," Ms. Bensouda said. "Rather, they appear to have been committed as part of approved interrogation techniques in an attempt to extract 'actionable intelligence' from detainees."

She said there is a "reasonable basis" to believe that the alleged U.S. war crimes "were committed in furtherance of a policy or policies aimed at eliciting information through the use of interrogation techniques involving cruel or violent methods which would support U.S. objectives in the conflict in Afghanistan."

The United States is not a member of the ICC, but its soldiers and agents could be subject to the court's investigation because their alleged crimes were committed in Afghanistan, which is an ICC member.

In addition, the CIA is alleged to have tortured some Afghan detainees in secret detention facilities in Poland, Romania and Lithuania, which are members of the ICC, the prosecutor's report said.

It said U.S. soldiers "appear to have subjected at least 61 detained persons to torture, cruel treatment [and] outrages upon personal dignity" in Afghanistan between 2003 and 2014, while CIA agents "appear to have subjected at least 27 detained persons to torture, cruel treatment, outrages upon personal dignity and/or rape" from 2002 to 2008 in Afghanistan, Poland, Romania and Lithuania.

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The victims of the alleged U.S. torture were "deliberately subjected to physical and psychological violence … committed with particular cruelty and in a manner that debased the basic human dignity of the victims," the prosecutor said.

"Some victims reportedly exhibited psychological and behavioural issues, including hallucinations, paranoia, insomnia, and attempts at self-harm and self-mutilation. The gravity of the alleged crimes is increased by the fact that they were reportedly committed pursuant to plans or policies approved at senior levels of the U.S. government, following careful and extensive deliberations."

The Afghan security forces, meanwhile, allegedly committed crimes that were "particularly gruesome and seemingly calculated to inflict maximum pain," the prosecutor said.

"The alleged crimes had severe short-term and long-term impacts on detainees' physical and mental health, including permanent physical injuries."

About 5,000 conflict-related detainees are still in Afghan government custody, and up to half are ill-treated, yet only two officials have ever been prosecuted for abusing detainees, the prosecutor said.