The "highest levels" of Sri Lanka's government were complicit in raping, torturing and abducting ethnic Tamils following the nation's ethnic civil war, a report by rights groups warned on Friday.
Authorities carried out horrific sexual abuse on Tamils, including forced oral sex and anal rape as well as water torture, the report by the UK Bar Human Rights Committee and International Truth & Justice Project found.
It comes ahead of next week's UN Human Rights Council debate on a US-led resolution calling for an international probe into alleged war crimes in Sri Lanka.
Yasmin Sooka, a top South African human rights lawyer and UN adviser, compiled the report from interviews with 40 Tamils who said they were held in custody by Sri Lankan authorities.
"Sri Lanka's military is still waging a campaign of persecution using abduction, arbitrary detention, torture, rape and sexual violence" after the crushing of separatist Tamil rebels in 2009, Sooka said.
Post-war, "widespread and systematic violations by the Sri Lankan security forces occur in a manner that indicates a coordinated, systematic plan approved by the highest levels of government", Sooka said.
The report, titled "An Unfinished War: Torture and Sexual Violence in Sri Lanka 2009-2014", included medical and psychiatric examinations of Tamils who sought refuge in Britain after being subjected to abuse in Sri Lanka.
"Their credible accounts, documented by nine independent lawyers from Western and Asian countries, establish a prima facie case to answer for post-war crimes against humanity involving torture, rape and sexual violence by the Sri Lankan military," she said.
Sooka said rights violators in Sri Lanka are still confident they will not be prosecuted.
Colombo has repeatedly denied its troops killed Tamil civilians while crushing Tamil Tiger rebels who at the height of their power controlled nearly a third of the island's territory.
The UN estimates 100,000 people perished during Sri Lanka's 37-year ethnic conflict.
Rights groups have accused government forces of killing up to 40,000 people in the final months of fighting in 2009.
South African rights activist Archbishop Desmond Tutu in a foreword to the report said it showed the government had not moved to ensure reconciliation between island's Tamil minority and majority Sinhalese population.
"My deepest hope is that the cycle of revenge will be broken," the archbishop said. "In order for this to happen, the international community must intervene.