Amnesty accuses US-backed Syria Kurds of torture, other abuses
The London-based watchdog said Washington too "likely" violated the human rights of detainees in Kurdish custody.
Camps and prisons run by the Kurdish-led Syrian Democratic Forces (SDF) still hold more than 56,000 detainees with alleged or perceived links to the Islamic State group, five years after the jihadists were ousted from their last territory in Syria.
They include jihadist suspects locked up in prisons, and wives and children of IS fighters held in the Al-Hol and Roj internment camps.
Kurdish authorities "have committed the war crimes of torture and cruel treatment, and likely committed the war crime of murder", Amnesty's secretary general, Agnes Callamard, said.
"The US government is supporting" the detention system and in some cases "is committing violations itself", an Amnesty report said, concluding that Washington "likely violated the human rights of many of those who have been within its effective control".
Al-Hol is the largest internment camp in northeastern Syria, with more than 43,000 detainees from 47 countries, many of them family members of IS fighters.
Kurdish authorities have long urged foreign governments with nationals in the camps to repatriate their citizens but Western governments have responded slowly for fear of domestic backlash.
Amnesty said some 94 percent of residents at Al-Hol and Roj were women and children, noting "no one in these camps has been charged or given the opportunity to challenge their detention before an independent judicial authority".
The camps and prisons likely hold thousands of human trafficking victims, including women and girls forcibly married to IS members, it said.
An AFP investigation at Al-Hol found residents lived in dire conditions, with evidence of child sexual abuse.
Amnesty said torture was carried out "systematically" in SDF-run prisons, including "severe beatings, stress positions, electric shocks and gender-based violence".
It said US forces had access to the detainees, noting that Washington had provided "hundreds of millions of dollars" to the SDF over the years.
A former detainee told Amnesty that US soldiers "checked on the prison, and they searched us, and all of our rooms... They could see the people who were injured from torture".
Callamard said: "The US government has played a central role in the creation and maintenance of this system... and must play a role in changing it."
She called on the US government, the Kurdish-led authorities and the United Nations to bring the "shameful" detention system "into compliance with international law".