A United Nations report says widespread shelling by the Sri Lankan government killed most of the civilians who died in the final months of the civil war there, in 2009.
The report also accuses Tamil Tiger separatists of using civilians as human shields and calls for an independent investigation into what it says could constitute war crimes.
It also criticises the UN itself for failing to take actions that might have protected civilians in the conflict, which lasted 25 years.
Sri Lanka had asked the UN not to publish its findings, saying the report could damage reconciliation efforts.
The BBC reports that the government has consistently denied allegations that it targeted civilians, and has rejected the report's findings as biased and fraudulent.
Secretary-General says he cannot intervene
UN Secretary-General Ban Ki-moon says he does not have the authority to order an investigation into the mass killings of civilians as the UN report has recommended .
A human rights group has disagreed with Mr Ban's description of his limited powers, saying he has the authority to push ahead.
But Mr Ban says he cannot automatically follow the recommendation because it would require either Sri Lanka's approval or a decision by UN member states.