Torture, abductions and summary killings of activists, critics, lawyers, ethnic and religious minorities by Syrian opposition groups Nusra Front, Ahrar ul Sham, Nureddin Zinki, Levant Front and Division 16 recorded in Amnesty report.
A report released by Amnesty International on Tuesday has accused opposition groups fighting in Syria of abuses ranging from torture and abductions to summary killings.
Calling the crimes "chilling", the report said methods employed by some of the groups in opposition-held areas of the northern Idlib and Aleppo provinces mimicked those of Bashar al Assad's regime.
According to the report, which documented cases of abuse over the course of four years based on the accounts of some 70 individuals living or working in rebel-held territory, some US-backed groups are among the perpetrators.
In total, five armed groups including the Al Qaeda-affiliated Nusra Front were accused of being behind a number of crimes.
The report documents at least 24 abductions of activists, critics, lawyers, ethnic and religious minorities, as well as three children, two of whom remain missing as of last week.
Public killings by gunfire were also said to have been carried out against captured pro-regime fighters, constituting war crimes, the report said.
Those who had been abducted—some simply for playing music—said they experienced methods of torture typically used by regime forces, including being suspended by the wrists for hours and being squeezed into tyres.
Amnesty's Middle East programme director Philip Luther said increasing cases of armed groups taking the law into their own hands had caused civilians, who welcomed an escape from "brutal" regime rule, to lose hope in having their rights respected.
"Many civilians live in constant fear of being abducted if they criticise the conduct of armed groups in power or fail to abide by the strict rules that some have imposed," Luther said.
"In Aleppo and Idlib today, armed groups have free rein to commit war crimes and other violations of international humanitarian law with impunity."
Amnesty urged the international community to "pressure armed groups to end such abuses and comply with the laws of war," adding the supply of arms and support to such groups should cease.
While one of the the groups accused in the report, Ahrar al Sham, said in a letter that it would like to meet Amnesty officials to clarify the issues, it did not respond to the allegations.
Other groups implicated in war crimes—Nureddin Zinki, the Levant Front and Division 16—are yet to offer any kind of response.
At least a quarter of a million people have died in the Syrian conflict since it started in March 2011 according to official UN estimates, while others placed the death toll at over 470,000.
The conflict has also displaced around half of the country's population. Approximately 7 million have been internally displaced while an estimated 5 million Syrians have sought refuge abroad.
Amnesty previously accused the Assad regime of being behind enforced disappearances as well as deliberately and systematically targeting hospitals and other medical facilities in northern Aleppo.
It also accused the PKK-affiliated YPG of forcing thousands of civilians out of their homes and demolishing entire villages in northern Syria, as well as threatening civilians with US-led air strikes if they fail to leave their homes.