As pressure grew for an international investigation into the killing of "tens of thousands" in the conflict's brutal finale, Ban was asked why the United Nations had played down casualty figures at the time.
The UN secretary general indicated that the global body could not have known the true number of dead in 2009 because staff had been withdrawn after the Sri Lankan government refused to guarantee their safety.
A UN panel this week said the Sri Lankan army and Liberation Tigers of Tamil Eelam guerillas may have committed war crimes in the conflict. But the three member panel also said the UN should investigate the actions of its own officials and agencies.
They said that the UN could have saved lives by putting more pressure on the government by highlighting the true number of dead.
Ban has agreed to the review. But he said: "At the time, the security situation was very precarious at the last stage of the crisis.
"And we were told by the Sri Lankan government, as I understa