While Sri Lankans are engaging in a process of reckoning and reconciliation, the UN has engaged in "self-scrutiny," Ban Ki-moon said on Friday, during a three-day visit to Sri Lanka.
He said had if the UN had been more active during Sri Lanka's civil war, many lives could have been saved.
"Sri Lanka has taught us many important lessons. (and) You have also made serious problems among your people," he said in a speech, adding that the UN made "big mistakes" during the critical last several months of the civil war.
An experts' panel appointed by Ban had reported that up to 40,000 ethnic Tamil civilians may have been killed largely due to shelling by government troops in the final months of the fighting, which ended in May 2009 with the defeat of the rebel Tamil Tigers, according to AP.
The rebels had been fighting for an independent state for ethnic Tamil minority complaining of systematic marginalization by successive governments controlled by majority Sinhalese.
Ban said Sri Lanka was only the latest of a series of UN failures, mentioning the 1994 genocide in Rwanda for which the UN "felt responsible."