An investigation into the last months of Sri Lanka's bloody civil war released yesterday claims government forces were responsible for the deaths of tens of thousands more civilians than estimated, and targeted hospitals and humanitarian operations as part of their final onslaught on the rebel Tamil Tigers.

According to the International Crisis Group study, many thousands more people may have died in the operation than United Nations figures suggested, with as many as 75,000 citizens unaccounted for, and almost all of the deaths in the so-called "No-Fire Zone" because of government fire.

The study also claims that the Government shelled hospitals where it knew international NGO staff and civilians to be working or receiving treatment.

"The Sri Lankan Government committed war crimes with top government and military leaders potentially responsible," it says. "An international inquiry into alleged crimes is essential."

The Sri Lankan Government has refused to comment on the report, the most comprehensive account of the violence that ended a year ago yesterday. Senior officials have insisted in the past there were no civilian casualties in the last months of the war.

At the weekend the Sunday Observer newspaper, widely considered a government mouthpiece, claimed that the report was part of a plot to promote former Army head Sarath Fonseka at the expense of President Mahinda Rajapaksa.

Fonseka was imprisoned after he lost the post-war election to Rajapaksa, accused of participating in political activity while still in uniform.

Yesterday the Government gave details of its "reconciliation commission", which would suggest methods for promoting national unity and determine compensation for those affected by the war with the Tamil Tigers, or Liberation Tamil Tigers of Eelam (LTTE).

The proposals stop short of any investigation into violations of humanitarian law. "The only chance of credible scrutiny is by outsiders," said Alan Keenan, Crisis Group's Sri Lanka project director.

United States Permanent Representative to the UN Susan Rice last week welcomed Sri Lanka's planned reconciliation commission, but cautioned it did need "to probe violations of international standards during the final stages of the conflict".

The civil war in Sri Lanka reached its zenith in early 2009, when the Government pinned the rebels down on the country's northern coast. The death toll has always been murky because reporters and independent observers were barred from the area.

Slim hopes of a UN-led inquiry now rest with the Secretary-General Ban Ki Moon, since the Security Council and Human Rights Council have declined to take Sri Lanka to task.

According to Crisis Group's president Louise Arbour, the lack of consequences for Rajapaksa makes it more likely other conflicts will be conducted in similar style in future, and provides no incentive for the draconian Sri Lankan regime to change its approach.