GENEVA - Syrian government forces and allied militias have raped and sexually assaulted women, girls and men in a campaign to punish opposition communities - acts that constitute war crimes and crimes against humanity, U.N. investigators said on Thursday.In a major report, they found that opposition groups in Syria's protracted civil war had also committed crimes of sexual violence and torture although these were "considerably less common."The report also said Islamic State (IS) and other armed extremist groups have executed women, men and children on charges of adultery, forced girls into marriage and persecuted homosexuals.The 29-page report by the U.N. Commission of Inquiry, issued as Syria enters its eighth year of war, is based on 454 interviews with survivors, relatives, eyewitnesses, defectors, lawyers and medical staff.Government forces raped civilians of both sexes during house searches and ground operations in the early stages of the conflict, and later at checkpoints and detention facilities, it said. The youngest known victim was a nine-year-old girl."Rape of women and girls was documented in 20 government political and military intelligence branches, and rape of men and boys was documented in 15 branches," the report by the U.N. war crimes investigators said.The independent experts led by Paulo Pinheiro - who have compiled confidential lists of suspects since 2011 - did not name individual perpetrators but said they had documented "numerous" cases of rapes by high-level officers.Branches where rapes took place included Aleppo, Deraa, Homs, Hama and Damascus, as well as Sednaya military prison and the air force intelligence branch at Mezzeh military airport, both near the capital, the report said."Sexual violence against females and males is used to force confessions, to extract information, as punishment as well as to terrorize opposition communities," the report said.Victims suffered shame, depression, incontinence, impotence and miscarriages, and rejection by their families, it said.The investigators found "no evidence of a systematic practice" on the part of armed groups to use sexual and gender-based violence to instill fear, but said that incidents had occurred in the context of sectarianism or revenge attacks.Throughout Syria's conflict, U.N. investigators had received allegations of extremist and terrorist groups imposing "medieval punishments on men accused of homosexuality."Women accused of adultery have been killed by stoning by the Islamist Jabhat Fateh al-Sham group, while homosexual men were thrown off a building by Jabhat al-Nusra jihadists, according to the report.