Geneva - The United Nations human rights chief thanked Tuesday the witnesses who came forth and gave testimony during the war crimes tribunals for the genocide in Rwanda 15 years ago.

"I would like to pay homage to the witnesses," said Navi Pillay, who was a judge on the International Criminal Tribunal for Rwanda. She said the "truth fosters reconciliation."

Pillay was speaking at an observance ceremony at the UN's Geneva headquarters, before an inauguration of an exhibition called A 100 Nights, which focused on the period of the genocide.

At least 800,000 Tutsis and moderate Hutus were killed during the mass slaughter in Rwanda in 1994.

The criminal tribunals were set up shortly afterwards and led to the first indictment against a head of government for his role in a genocide. Jean Kambanda, prime minister during the genocide, was given a life sentence in 1998, which he serves in Mali.

Pillay quoted the British 18th century thinker Edmund Burke, that evil triumphs when good men to do nothing, stressing the importance of intervention both before and after a serious crime.

However, she said the she hoped the International Criminal Court in the Hague would be "used to prevent war crimes, rather than prosecute heinous deeds after the fact."

She noted that Rwanda became the first case since the post World War II trials in which perpetrators of the worst crimes were held accountable, ending what she called 50 years of impunity.

"Genocide is the ultimate form of discrimination and racial hatred," Pillay said, calling for states to attend the upcoming conference against racism in Geneva, scheduled for later this month.

The war crimes tribunals for Rwanda were still engaged in work and while they were set to be done by 2010 this date is now seen as unlikely. (dpa)