The United Nations Secretary General Kofi Annan has said he could and should have done more to stop the genocide in Rwanda 10 years ago.
Annan headed UN peacekeepers at the time of 1994 genocide
At a memorial conference at the UN, Mr Annan said he realised he personally could have done more to rally support for international efforts to stop it.
"The international community is guilty of sins of omission," Mr Annan said.
The genocide - in which some 800,000 people died - occurred when Mr Annan was head of UN peacekeeping forces.
The UN Security Council failed to reinforce the small UN peacekeeping force in the country.
"The international community failed Rwanda and that must leave us always with a sense of bitter regret," Mr Annan said.
He said the painful memory had influenced many of his later decisions as secretary general.
"I believed at that time that I was doing my best," he said.
"But I realised after the genocide that there was more that I could and should have done to sound the alarm and rally support."
Rwandans have accused the peacekeepers who were there at the time of failing to protect them.
One survivor, Specios Kenya Bugoi, described how 4,000 Tutsis took shelter close to Belgian troops hoping to be safe.
Speaking through an interpreter she described how the troops left and the killings began.
1994: RWANDA'S GENOCIDE
April: Rwandan Hutu president Habyarimana killed when plane shot down
April -July: An estimated 800,000 Tutsis and moderate Hutus killed
July: Tutsi-led rebel movement RPF captures Rwanda's capital Kigali
July: Two million Hutus flee to Zaire, now the DRC
"During that massacre I lost my husband, members of my family, all of my friends, neighbours," she said.
"I slept among the cadavers for the whole night."
In April 2000 the UN Security Council admitted responsibility for failing to stop the genocide.
The Canadian Foreign Minister, Bill Graham, told the conference that 10 years after the genocide the international community had still not learned how to stop such killings from happening again.
"We lack the political will to achieve the necessary agreement on how to put in place the type of measures that will prevent a future Rwanda from happening," he said.
The head of the small UN peacekeeping force in Rwanda at the time, Lieutenant General Romeo Dallaire, told the conference that no-one was interested in saving Rwandans and the bulk of his force was ordered to leave.
He suggested that attitudes now had not changed.
"I still believe that if an organisation decided to wipe out the 320 mountain gorillas there would be still more of a reaction by the international community to curtail or to stop that than there would be still today in attempting to protect thousands of human beings being slaughtered in the same country."
The UN has designated 7 April as international day of reflection on the genocide.
Mr Annan announced he was backing a call from the Rwandan government for the world to observe a minute of silence to remember the victims and resolve to prevent such a tragedy from ever happening again.
"Let us be united in a way we were not 10 years ago," he said.