An internal United Nations report says the organisation was responsible for a grave failure to protect civilians in the final stage of Sri Lanka's civil war three and a half years ago.
It says senior UN staff in the capital, Colombo, did not see the prevention of the killing of 40,000 civilians as their responsibility.
The report said the UN should in future "be able to meet a much higher standard in fulfilling its protection and humanitarian responsibilities".
The war lasted 26 years and left at least 100,000 people dead. The government and Tamil rebels are accused of war crimes in the conflict, which ended in May 2009.