The hands of the United Nations are stained with blood over its failure to stop the atrocities 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 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 thaw in recent years.
The Prime Minister 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."
His condemnation came as the Prime Minister announced an additional 12 million US dollars (£16.7 million) of British humanitarian aid for civilians caught up in the civil war in Syria.
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.
He urged the international community to keep faith in the process of change in the Arab world, while accepting progress would be slow in some areas.
"Nothing in the last year has changed my fundamental conviction," said Mr Cameron.
"The Arab Spring represents a precious opportunity for people to realise their aspirations for a job, a voice and a stake in their own future.
"And we, in this United Nations, must do everything we can to support them."
Mr Cameron said: "One year on, some believe that the Arab Spring is in danger of becoming an Arab Winter.
"They point to the riots on the streets, Syria's descent into a bloody civil war, the frustration at the lack of economic progress and the emergence of newly elected Islamist-led governments across the region.
"But they are in danger of drawing the wrong conclusion.
"Today is not the time to turn back - but to keep the faith and redouble our support for open societies, and for people's demands for a job and a voice."
Describing himself as a "liberal Conservative not a neo-conservative", Mr Cameron said he had never expected democracy to provide a swift and easy solution to the area's problems, but respected each country's right to develop in its own way.
"We cannot expect the damage of decades to be put right in a matter of months," he said.
Mr Cameron said Britain was ready to work with recently-elected Islamist governments such as Egypt's, but stressed they would be judged on issues such as their treatment of women and Christian minorities, as well as whether they seek to interfere in neighbouring states.
"Democracy and Islam can flourish alongside each other," said Mr Cameron.
"So let us judge governments not by their religion - but by how they act and what they do.
"And let us engage with the new democratic governments in Egypt, Tunisia and Libya so that their success can strengthen democracy not undermine it."
He announced that a British Government taskforce has been set up to clear bureaucratic hurdles to the repatriation of assets of the former regimes in Arab Spring states, which were frozen during the last year's upheavals.
In private talks ahead of his address, Mr Cameron was urged by Egyptian president Mohamed Mursi to step up pressure on Russia and China to back a Security Council resolution on Syria.
In their 30-minute meeting, Mr Mursi agreed that the Assad regime must not remain in place, but that external military intervention would make matters worse.
He voiced his support for further EU, US and Arab sanctions on Syria, and said he wanted to explore how Arab neighbours can work together to increase pressure on Assad.
Mr Cameron will be heartened by this backing for his stance from the democratically-elected leader of the most populous Arab state.