The hands of the United Nations are stained with blood due to its failure to stop the atrocities of the Assad regime in Syria, Prime Minister David Cameron has told the international body's General Assembly.
In a clear challenge to Russia and China - which have blocked Security Council resolutions on Syria - Mr Cameron used his keynote address to the General Assembly to call for the whole international community to support moves to bring about a transition of power in Damascus.
He cited a recent Save the Children report documenting the torture and murder of children by regime forces in Syria.
And he told world leaders gathered at the UN headquarters in New York: "The blood of these young children is a terrible stain on the reputation of this United Nations. And in particular, a stain on those who have failed to stand up to these atrocities and in some cases aided and abetted Assad's reign of terror. If the United Nations Charter is to have any value in the 21st century, we must now join together to support a rapid political transition."
Mr Cameron's strongly-worded comments risk worsening relations with Moscow after a gradual thaw over recent years.
The PM also announced an additional £16.7 million of British humanitarian aid for civilians caught up in the civil war in Syria.
Mr Cameron made clear he also laid some of the blame for atrocities in Syria at the door of Iran, which has backed president Bashar Assad.
"Assad has colluded with those in Iran who are set on dragging the region into wider conflict," said the PM. "The only way out of Syria's nightmare is to move forward towards political transition and not to give up the cause of freedom. The future for Syria is a future without Assad. It has to be based on mutual consent as was clearly agreed in Geneva in June."
Mr Cameron's speech came a year after he told the same body that he welcomed the emergence of democratic movements across north Africa and the Middle East in the Arab Spring.
With Syria still mired in war, Islamist parties winning elections in Egypt and US ambassador Chris Stevens murdered by a mob in Libya earlier this month, Mr Cameron acknowledged that doubts over the Arab Spring were gaining ground in western capitals.