US Secretary-General Kofi Annan opened a memorial conference on the 1994 Rwanda genocide by accepting institutional and personal blame for the slaughter of 800,000 civilians that was initially ignored by world leaders.
"The international community is guilty of sins of omission," said Mr Annan, who was head of the United Nations peacekeeping agency at the time and had asked countries to provide troops.
"I believed at the time that I was doing my best. But I realised after the genocide that there was more that I could and should have done to sound the alarm and rally support," Mr Annan said in a speech to open the "Memorial Conference on the Rwanda Genocide."
It was not the first time that the secretary-general had criticised the United Nations and his own mistakes, but he said the painful memory of Rwanda and ethnic cleansing in Bosnia-Herzegovina in the mid-1990s "has influenced much of my thinking, and many of my actions" as head of the world body.
The small central African country was plunged into ethnic butchery in April 1994 after a plane carrying President Juvenal Habyarimana was shot down over Kigali.
Some 800,000 Tutsis and Hutu moderates were slain in about 100 days by Hutu extremists and their followers, armed with machetes, garden hoes and spiked clubs. They were spurred on by hateful radio broadcasts.
The genocide was halted when Tutsi-led rebels overthrew the Hutu extremists, many of whom fled to neighboring Zaire, now called the Democratic Republic of Congo. The rebels went on to form Rwanda's government.
Mr Annan has designated April 7th the "International Day of Reflection on the Genocide of Rwanda" and supported a request from the Rwandan government that the world observe one minute of silence on that day at 12 noon in each time zone.
"None of us must ever forget, or be allowed to forget, that genocide did take place in Rwanda, or that it was highly organized, or that it was carried out in broad daylight," Mr Annan said.