Amnesty International charged that rebel groups backed by the United States, Saudi Arabia, Qatar, and Turkey have been responsible for abductions, torture, and summary killings in Syria.
The London-based rights group on July 5 documented 24 kidnappings by armed groups in Aleppo and Idlib provinces between 2012 and 2016.
Victims included peace activists, children, Kurds, Christians, and other minorities targeted solely because of their religion, Amnesty said, with some groups using the same forms of torture practiced by President Bashar al-Assad's regime.
Rebel groups cited as committing the abuses included the Nour al-Din Zanki Movement, the Levant Front, the 16th Division, and hardline Islamist group Ahrar al-Sham.
The U.S.-based Institute for the Study of War said that the first three were current or former recipients of U.S. support.
"While some civilians in areas controlled by armed opposition groups may at first have welcomed an escape from brutal Syrian government rule, hopes that these armed groups would respect rights have faded as they have increasingly taken the law into their own hands and committed serious abuses," said Amnesty's Philip Luther.