Rebel fighters have committed similar violations, amounting to war crimes, but at a rate "considerably less common than rape by government forces and associated militia", the Independent International Commission of Inquiry for Syria (COI) said in a new report.
The findings, submitted to the United Nations Human Rights Council, are based on 454 interviews with sources that include survivors, eye witnesses and medical workers. The Damascus government has never granted COI investigators access to Syria.
Overall, the report adds to the overwhelming accounts of hellish suffering endured by Syrian civilians during the conflict that has claimed more than 350,000 lives as it enters its eighth year.
A woman from Syria's third city of Homs told COI investigators that in 2012 "government forces entered her home and raped her daughter in front of her and her husband before shooting the daughter and the father", the report said.
"The mother was then raped by two soldiers," it added, in one of many examples of extreme violence.
Checkpoints controlled by the government or its allies, as well as detention centres, were identified as a main areas where sexual violence was perpetrated.
The COI notes that government troops detained "thousands of women and girls" from 2011 to the end of 2017, the period covered in the report.
"During interrogation, women and girls... were beaten with pipes while hanging from the ceiling and electrocuted on their breasts and vaginas," it said.
While women have been the most frequent victims of rape, sexual violence against men and children -- girls and boys -- was also documented.
The COI found that "rapes and other acts of sexual violence carried out by government forces and associated militias during ground operations, house raids, at checkpoints, and during detention formed part of a widespread and systematic attack directed against a civilian population, and amount to crimes against humanity."
In contrast, it found "no evidence of a systematic practice or policy on the part of armed (rebel) groups to use sexual and gender-based violence to instil fear, extract information, or enforce loyalty," even if the opposition members have repeatedly committed sexual violence.
That finding does not apply to the Islamic State group, whose widespread atrocities including sexual violence have been separately documented by the COI.
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