The report, which was made public today, concluded that tens of thousands of civilians were killed in northern Sri Lanka from January to May 2009 and that the Sri Lankan Government knowingly shelled areas where it had encouraged civilians to gather.
The report gives credibility to allegations that both the government and the Liberation Tigers of Tamil Eelam (LTTE) committed serious violations of international humanitarian law and international human rights law.
"Almost two years after the end of the conflict, this UN report finally exposes the Sri Lankan government's whitewash in its efforts to deny justice to the war's victims," said Sam Zarifi, Amnesty International's Asia-Pacific Director.
"UN Secretary-General Ban Ki-moon must ensure that the UN established a commission of inquiry to collect evidence on the alleged crimes by both sides, to determine who did what to whom, and to recommend next steps for bringing suspected perpetrators to justice in a transparent and timely manner."
The report also adds weight to other allegations made since the conflict ended.
These include claims that the Sri Lankan government deliberately underestimated the number of civilians remaining in the conflict zone and systematically deprived them of humanitarian aid, including food and medical supplies.
The LTTE recruited child soldiers, held civilians hostage, using them as human shields, and shot people who attempted to escape, the UN panel found.
"Eyewitness accounts by survivors of the final months of fighting paint a very grim picture," said Sam Zarifi.
"They lived in profound fear, suffering injuries and loss of life, and were deprived of food, water and medical care. Many of those who finally escaped the conflict zone were detained by the army in miserable conditions; some remain in detention without trial two years later. How can we deny them justice now?"
In a statement posted on a state news agency website on 21 April, the Sri Lankan government called on the UN not to release the report and rejected its findings.
China, Russia and other states that supported the Sri Lankan government's campaign against the LTTE have blocked moves at the UN to consider alleged war crimes during the conflict, and joined Sri Lanka in opposing the establishment in June 2010 of the Panel of Experts that produced the report.
These states have looked to a Sri Lankan government-established Lessons Learnt and Reconciliation Commission as a possible substitute for an international commission of inquiry into war crimes.
The UN report bolsters Amnesty International's position that the national Commission is not impartial and has no mandate or will to investigate and prosecute the alleged crimes.
"It is time for the governments that have obstructed international scrutiny of the crimes to step aside now. The many other governments who have remained disturbingly silent must now come forward and demand justice for the conflict's victims," said Sam Zarifi.
Amnesty International also calls on the national authorities of other countries to exercise universal jurisdiction to investigate crimes identified in the report and to prosecute them in their national courts, where appropriate.
"An international inquiry, especially into the violations committed by the LTTE, will greatly help the process of reconciliation in Sri Lanka," added Sam Zarifi