Syrian government forces have used sexual violence to torture men, women, girls and boys detained during the current conflict, Human Rights Watch said on Friday.
The New York-based group said it had interviewed 10 former detainees, including two women, who described being sexually abused or witnessing such abuse in detention.
This included "rape, penetration with objects, sexual groping, prolonged forced nudity and electroshock and beatings to genitalia," a statement said.
Many witnesses said they were were imprisoned because of their political activism, while others said the reasons for detention were unclear but that detainees suffered the same abusive tactics.
"Sexual violence in detention is one of many horrific weapons in the Syrian government's torture arsenal and Syrian security forces regularly use it to humiliate and degrade detainees with complete impunity," said Sarah Leah Whitson, HRW's Middle East director.
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"The assaults are not limited to detention facilities. Government forces and pro-government shabiha militia members have also sexually assaulted women and girls during home raids and residential sweeps."
HRW said the full extent of such abuse in and outside of detention facilities remains unknown, as the stigma in Syria surrounding sexual violence makes victims reluctant to report abuse.
"In many cases interviewees told Human Rights Watch that victims did not want their families or others in the community to know about the assault because of fear or shame," the statement said.
HRW said victims have limited access to medical or psychological treatment and other services at home, or even as refugees in neighbouring countries, which is exacerbated by "taboos surrounding sexual abuse, families restricting their movement and the fear of being subjected to so-called 'honor' crimes."
The watchdog said it did not have evidence that high-ranking officers ordered troops to commit sexual violence, but many of the reported assaults "were in circumstances in which commanding officers knew or should have known the crimes were taking place," such as in detention centres.
But it said information, including from defectors, "indicates that no action has been taken to investigate or punish government forces and shabiha who commit acts of sexual violence or to prevent them from committing such acts."