UN leader Ban Ki-moon says the failure to halt atrocities in Syria has become a new stain on the reputation of the world body.
"Our collective failure to prevent atrocity crimes in Syria of the past two-and-a-half years will remain a heavy burden on the standing of the United Nations and its member states," Ban told a UN meeting on preventing genocide.
Speaking ahead of a new bid by Russia and the US to head off a possible military strike on Syria, Ban said the UN Security Council must play "an effective role in promoting an end to the Syrian tragedy".
Ban said world leaders had vowed to act to prevent a repetition of the Rwanda genocide in 1994 and 1995 massacre in the Bosnian city of Srebrenica.
The Security Council is widely blamed for failing to act over the horrific events. And the 193-country UN General Assembly agreed the "responsibility to protect" doctrine in 2005, which has since been used to back UN military mandates for crises in Libya and Ivory Coast.
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"As we see around us, however, atrocities continue to be committed," Ban said highlighting the Syria case.
"Many observers regard the international community's divisions and immobility as a failure of the responsibility to protect," he added, while insisting that the doctrine remains an important weapon.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are to meet in Geneva on Thursday in a bid to agree Security Council measures against the use of chemical weapons in Syria.
Moscow, backed by China, has vetoed three UN Security Council resolutions proposed by Western nations that aimed to increase pressure on Assad.
