The report was produced by an independent panel of experts studying accountability issues arising from alleged human rights violations committed by both sides during the last phases of the conflict, which ended a protracted civil war in the South Asian country in May 2009.
"The panel found credible allegations, which if proven, indicate that a wide range of serious violations of international humanitarian law and international human rights law were committed both by the government of Sri Lanka and the LTTE, some of which would amount to war crimes and crimes against humanity," said excerpts of the report published in the Island Newspaper in Sri Lanka.
"Tens of thousands lost their lives from January to May 2009, many of whom died anonymously in the carnage of the final few days," the report said.
Following the leak, which the U.N. has confirmed as accurate, Sri Lankan President Mahinda Rajapaksa has called for a "show of strength" against the findings.
The Sri Lankan government described the report as "fundamentally flawed in many respects" and "based on patently biased material, which is presented without any verification."
The summary of the report was leaked after the U.N. sent a copy of it to the government.
Observers have said that the leak before the report has been officially released has undermined its impact as well as bolstered the government's opposition against it.
"We certainly regretted the leaking of this report," U.N. deputy spokesman Farhan Haq told journalists on Tuesday. "We are aware of the consequences of this leak, and we've already made clear our dismay at that."
The U.N., which is reviewing the report, has said it hopes to release it along with the Sri Lankan government's response. The report will be out later this week. Meanwhile, the Island newspaper is publishing more excerpts.
Mathew Lee, a journalist who has been closely following the situation in Sri Lanka, said that the U.N. should have released the report as soon as the excerpts were leaked to the media.
"Whether intentionally or unintentionally the U.N. has allowed the government to give its own spin to the report," he said.
The report also criticized the Sri Lankan government's attempts to investigate. It concluded that the "government's notion of accountability is not in accordance with international standards."
The panel further said that the Lessons Learnt and Reconciliation Commission, formed by the government, had not "conducted genuine truth-seeking about what happened in the final stages of the armed conflict."
After the conflict ended in 2009, a wave of relief washed over the Sri Lankan population, which had suffered the consequences of a 27-year-long civil war that killed an estimated 80,000 to 100,000 people.
The Tamil Tigers held grievances that minority ethnic Tamils in the country had been discriminated against by the majority ethnic Sinhalese. The separatist group, which wanted a homeland for the Tamils in the north and east of the country, had terrorized the population with attacks on civilians using bombs and suicide bombers as well as recruiting child soldiers.
Months after the military action ended, however, allegations of widespread human rights abuses began to emerge against the government.
For instance, an independent expert concluded that a tape showing a Sri Lankan soldier shooting a bound and blindfolded Tamil rebel at point-blank range was authentic.
"The result of this analysis then would seem to point to the need for the government of Sri Lank to undertake the investigation that I had called for initially," Philip Alston, the former U.N. Special Rapporteur on extra judicial killings, said in 2010.
But the Sri Lankan government has consistently rejected any international perusal of its human rights record. The panel members were not allowed to enter the country despite the U.N. repeatedly describing it as an "advisory panel" that would not be fact-finding or investigating.
The U.S. has encouraged the Sri Lankan government to "use" the report in seeking accountability. "We believe also that the U.N. Panel of Experts is a useful asset and should be taken advantage of by the government," State Department spokesman Mark Toner said recently.
It is not clear yet whether the report will lead to action if the government refuses to accept it. Following this initial report, human rights groups are likely to urge Secretary General Ban Ki-moon to set up a more comprehensive investigation.
Further investigation, however, could require the approval of other U.N. countries. But governments, which generally don't favor international intervention, could block more action.
Responding to whether the situation may head toward a dead-end, Meenakshi Ganguly, South Asia director of Human Rights Watch, said that it depended on how much pressure could be put on Sri Lanka.
The human rights expert noted that the present discourse had become about national sovereignty instead of accountability and justice, which needed to be corrected.
"It is a country that needs some closure," she said.