Secretary-General Ban Ki-moon has commemorated the 15th anniversary of the Serb massacre of more than 8,000 Muslims taken from a UN enclave by vowing never again to allow a similar atrocity.
The slaughter in the eastern Bosnian town of Srebrenica - the worst atrocity in Europe since the Second World War - is a stain on the history of the United Nations, he said.
"And, we vow, together, never again to allow such an atrocity to happen at any time, in any place," Mr Ban said. "This we owe to the souls of Srebrenica. This we owe to our common humanity."
On July 11, 1995, some 30,000 Bosnian Muslims flocked to the UN military base in the town's suburb for refuge. But outnumbered Dutch UN peacekeepers allowed Serb forces inside the gates.
The Serbs then separated out the Muslim men and boys, putting them on trucks and carting them away, the vast majority never to be seen again.
The UN Security Council began its meeting on Monday with a minute of silence in commemoration of the Srebrenica victims.
Mr Ban said the United Nations cannot undo the past but he said the world body must learn from Srebrenica.
"We must remain steadfast in ensuring that humankind never forgets those lessons," he said.
"The work of the International Criminal Court, our efforts to protect civilians, our increased vigilance for early signs of genocide or other grave crimes are all meant to reduce the risk of another such assault on innocents - and to fully prepare us if it does come."