Amnesty International has accused an al-Qaeda-linked group that controls large areas of northern Syria of torturing and killing detainees in secret prisons.
The Islamic State of Iraq and the Levant (ISIL) has "ruthlessly flouted the rights of local people", the rights group said in a report released on Thursday.
"Those abducted and detained [by ISIL] include children as young as eight who are held together with adults in the same cruel and inhuman conditions,” said Philip Luther, Amnesty International’s Director for the Middle East and North Africa.
The report described individuals being seized by masked men, held for weeks on end in solitary confinement at unknown locations and tried by self-styled Sharia courts that mete out death or floggings with little if any due process.
It cited former detainees held in Raqqa and Aleppo provinces describing how they were flogged with rubber generator belts or cables, tortured with electric shocks or forced to adopt a painful stress position known as the scorpion, in which a detainee’s wrists are bound over one shoulder.
Amnesty said some people were detained for common crimes like theft while others were jailed for smoking, sex outside of marriage, or because they challenged ISIL's rule or belong to other armed groups.
It said a child of about 14 accused of stealing a motorbike was repeatedly flogged over several days.
The London-based group called on ISIL to end its "appalling treatment of detainees and for the group’s leaders to instruct their forces to respect human rights and abide by international humanitarian law".
It also called on Turkey and Gulf states - which support mainstream rebel groups - to take measures to prevent the flow of arms and aid to ISIL and other groups accused of human-rights violations.
In recent months, ISIL has kidnapped dozens of Syrian activists and news providers, as well as several foreign journalists.
ISIL has carried out scores of suicide bombings and other attacks in both Syria and Iraq.
It is believed to include large numbers of foreign fighters and aims to create an Islamic state that would supersede national borders.
Both sides in Syria's war have been accused of gross human-rights violations.
The UN General Assembly on Wednesday approved a resolution expressing outrage at "widespread and systematic gross violations" by Syrian authorities.
The resolution demands an end to all human-rights abuses, the immediate release of all detainees and immediate steps by the Syrian government to expand humanitarian relief operations.
More than 120,000 people have been killed since the uprising against the government of President Bashar al-Assad began in March 2011.