The panel, led by former UN official Charles Petrie criticised what it termed as "a sustained and institutionalised reluctance" by staff members in Sri Lanka at the time "to stand up for the rights of the people they were mandated to assist".
"Many senior UN staff simply did not perceive the prevention of killing of civilians as their responsibility," the report's executive summary said.
Finding fault with the way the crisis was dealt with by senior UN officials in New York, the draft report said, "Decision-making across the UN was dominated by a culture of trade-offs –from the ground to UN headquarters".
Officials chose "not to speak up" about "broken commitments and violations of international law" by both the Sri Lankan government and Tamil Tiger rebels because that "was seen as the only way to increase UN humanitarian access" to victims of the conflict, a report in the New York Times said.
"The last phase of the conflict in Sri Lanka presented a major challenge" to the international body, it added.