Sri Lankan security forces were responsible for the ‘‘vast majority’’ of alleged war crimes during the final months of its bloody civil war, and may have systematically destroyed mass burial sites of civilians in an apparent effort to destroy evidence, a new report has found.

The major new study examines allegations of the deliberate and indiscriminate shelling of civilians corralled into no-fire zones by the Sri Lankan government. Tens of thousands of civilians are believed to have perished.

In addition, the report by the Public Interest Advocacy Centre’s [PIAC] international crimes evidence project investigates the alleged assassination of surrendering Tamil Tigers figures.

The Sydney Morning Herald reported that the rape and torture of civilians and the alleged shelling of a hospital and refusal to distribute food and medicines to civilians were also probed, along with the alleged use of civilians as human shields and child soldiers by the rebel Tamil Tigers, or Liberation Tigers of Tamil Eelam.

In all cases, the report finds there is credible evidence of war crimes and crimes against humanity and urges the UN to formally investigate the allegations after two UN resolutions for the Sri Lankan government to address them led to little action.

‘‘Although violations were committed by both sides, the evidentiary material indicates that members of the Sri Lankan security forces perpetrated the vast majority of alleged crimes during the investigation period [September 2008 to May 2009],’’ it said.

Sri Lanka’s government, which routed the Tamil Tigers, flatly denies any wrong-doing and is resisting an international investigation likely to be put forward as a resolution by the US to the UN Human Rights Council next month.

The report, titled Island of Impunity? and released on Wednesday, uses forensic analysis of photographs, satellite imagery, independent military analysts and new witness accounts to probe the litany of well-aired allegations.

In a disturbing new development, it spoke to three different witnesses involved in the registration and burial of dead civilians. One of the witnesses alleges that the Sri Lankan security forces went back to the burial sites after the conflict concluded and systematically destroyed human remains.

The US is expected to recommend formal investigation by the UN but whether Australia will co-sponsor or support any resolution remains unclear. Foreign Affairs Minister Julie Bishop did not respond to questions before deadline.

The Sydney Morning Herald further reported that the Australian government has close ties with the Sri Lankan government, with Prime Minister Tony Abbott giving two patrol boats to its navy to intercept asylum seekers and refusing to criticise its human rights record, praising it for bringing ‘‘more freedom and more prosperity’’ at a Commonwealth leaders’ meeting in Colombo in November.

At the same meeting, British Prime Minister David Cameron demanded an international investigation into the alleged war crimes.