A leading international watchdog on Thursday accused an al-Qaeda-linked rebel group that controls large parts of northern Syria of running secret prisons in which torture and summary killings are common.

The allegations by Amnesty International came just hours after the UN General Assembly approved a resolution condemning "widespread" violations by Syrian government forces.

In an 18-page report, the Britain-based watchdog said one of the most powerful armed groups on Syria's opposition side, known as the Islamic State in Iraq and the Levant, has "ruthlessly flouted the rights of local people" in territory that the group, along with moderate rebel factions, seized over the past two years from President Bashar al-Assad's forces.

Flogging anyone, let alone children, is cruel and inhuman, and a gross abuse of human rights . - Philip Luther, Amnesty's director for the Middle East and North Africa

Children as young as eight are held along with adults in seven ISIL-run detention facilities in Syria's Aleppo and Raqqa provinces, Amnesty said. Many are often held for challenging ISIL's rule, petty crimes like theft or for committing purported "crimes against Islam" such as smoking cigarettes.

Human rights groups have said both sides in Syria's nearly three-year-old conflict have violated the rules of war.

They've long claimed that Assad's government has arbitrarily detained tens of thousands of Syrians on suspicion they were opposition supporters. Abuses on the rebel side are growing, but have not reached the scale and intensity of those by the government, activists say.

Amnesty's findings were based on interviews with former detainees who detailed abuses they themselves experienced or provided accounts about what others held with them endured at the hands of the Islamic group, including flogging with rubber belts or generator cables, electric shocks and forced stress positions.

Secret prisons

Former detainees told Amnesty that they were abducted by masked gunmen, who took them to undisclosed locations, where they were held for up to 55 days. Some never learned where they had been held, but Amnesty said that based on their testimonies the watchdog identified ISIL prisons at three locations in the city of Raqqa that earlier this year became the first urban centre to have fallen entirely under opposition's control.

Amnesty also identified two other prisons in an oil facility in Raqqa province, and two detention facilities run by the Islamic group in the neighbouring Aleppo province.

Several children were among detainees who received severe floggings, according to testimonies obtained by Amnesty.

Two detainees told Amnesty they saw a teenager of about 14 flogged with more than 90 lashes during an interrogation by ISIL captors in one of the detention facilities in Raqqa. Another youth, also about 14, whom the al-Qaeda-linked group accused of stealing a motorbike, was repeatedly flogged over several days, Amnesty's report says.

"Flogging anyone, let alone children, is cruel and inhuman, and a gross abuse of human rights," said Philip Luther, Amnesty's director for the Middle East and North Africa.

In New York on Wednesday, the General Assembly approved a resolution expressing outrage at "widespread and systematic gross violations" by Syrian authorities. The resolution was initiated by Saudi Arabia on behalf of dozens of countries, including the United States.

And while General Assembly resolutions are not legally binding, they offer a strong expression of world opinion — in this case, the civil war in Syria, where nearly three years of fighting between the Syrian troops and opposition forces has killed more than 120,000 people.