Human Rights Watch warned European countries on Tuesday against transferring foreign jihadist suspects from prisons in war-torn northeastern Syria to Iraq.
The New York-based watchdog expressed concern that some countries with significant contingents of prisoners in Syria were seeking to move them across the border.
A Turkish invasion of areas controlled by Kurdish forces has sparked deep concern in Europe about the potential for mass breakouts by members of the Islamic State group.
Many European governments have been reluctant to repatriate these prisoners to stand trial and have already transferred some to Iraq to be prosecuted.
"Given Iraq's record of unfair trials, European states should not promote efforts to have their nationals transferred there for prosecution," HRW's Iraq researcher Belkis Wille said.
Any government supporting such a move "without taking measures to remove the risk of torture, sham trials and execution risks contributing to serious abuses," she said.
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Wille said her organisation's monitoring showed that trials in Iraq were "inherently unfair and replete with due process violations".
She urged Britain, Denmark, France, Germany and other countries to seek their nationals' repatriation instead.
The United States announced on Monday that an order to withdraw nearly all of its troops from Syria was already being implemented.
That leaves the Kurdish forces it once backed in the lurch and abruptly dismantles Western military capabilities against IS in the area.
According to the Kurdish administration in northeastern Syria, around 12,000 IS suspects are held in prisons across the area. Around 2,500 of them are non-Iraqi foreigners.
European officials have expressed fears that the US pullout decision and subsequent Turkish invasion could allow a resurgence of IS in the region and lead to more attacks in their own cities.