Sri Lankan security forces carried out arbitrary killings, torture and rape as they put down the final stages of a separatist insurgency, an independent investigation said .

The Australia-based Public Interest Advocacy Centre also said the military had tried to destroy evidence of mass civilian deaths during the country's drawn-out civil war with Tamil fighters.

"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," the investigation concluded.

The report, Island of Impunity, calls for an internationally mandated investigation into the allegations during the closing stages of the war in 2008 and 2009.

Sri Lanka has been under mounting pressure over alleged rights abuses and civilian killings during its battle with the Tamil Tigers.

The US is reportedly planning on bringing a third resolution to the UN human rights council calling on Sri Lanka to do more to investigate alleged war crimes.

"I see the allegations of war crimes attempted to be brought against Sri Lanka in Geneva as a victory for those who are opposed to peace," said Sri Lankan President Mahinda Rajapaksa on Tuesday.

He said it was a "grave crime" to make such accusations based on information provided by "separatists and losers."

Government troops have been accused of targeting civilians and hospitals, blocking food and medicine for civilians and deliberately undercounting the number of civilians trapped in the war zone.

Rebels have been accused of using civilians as a human shield.