David Cameron charged China and Russia with blocking international efforts to halt the war in Syria and even suggested they had "aided and abetted" President Bashar al-Assad's "reign of terror".
While Mr Cameron, in an address to the United Nations yesterday, did not mention them by name, it was clear he was alluding to Russia and China and their unwillingness on three different occasions to approve resolutions in the Security Council to pressure Assad to step down.
Perhaps more striking, however, was the allegation that, more than just stymying the Security Council, they have been assisting Assad's war on the rebels and the Syrian people. He did not specify whether he was referring to moral support or the delivery of arms.
Mr Cameron prefaced the attack by quoting the words of a 16-year-old Syrian called Wael, who was detained in a police station in the town of Deraa. "I have seen children slaughtered," he quoted the teenager as saying. "No, I do not think I will ever be ok again...If there was even 1 per cent of humanity in the world, this would not happen".
The Prime Minister said: "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."
While Mr Cameron was giving voice to the frustration over the bloodletting – which the UN Secretary General Ban Ki-Moon on Tuesday said had become "a regional calamity with global ramifications" – it was not clear whether his words would help lubricate the Security Council or smooth the way for William Hague, the Foreign Secretary, who is to meet his Russian counterpart, Sergei Lavrov, in New York today.
The grave tone of Mr Cameron was in stark contrast with an appearance planned for later last night on the popular variety show, "The Late Show with David Letterman", on the CBS network, before his departure for Brazil for the start of a two-day official visit.
Mr Cameron last night also offered Egypt fresh help to unfreeze assets stolen by the former regime of Hosni Mubarak, by sending a British Task force to Cairo and assist in the unravelling of European Union regulations. Hours before making his speech, he detailed the offer in a bilateral meeting with President Mohammed Morsi. "It is simply not good enough that the Egyptian people continue to be denied these assets long after Mubarak has gone," he proclaimed.
He used his time in New York to woo the new leaders of the Arab Spring, vowing to continue to support it and urging patience even as some claim that, with recent eruptions of violence notably against the United States in several Arab capitals, it is turning to an Arab Winter.
"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," he said. "Yes, the path is challenging. But democracy is not – and never has been – just about holding an election. It is not one person, one vote, once. We cannot expect the damage of decades to be put right in a matter of months."
The Prime Minister also paused to pay tribute America's ambassador to Libya who was killed earlier this month.. "The murder of Ambassador Chris Stevens was a despicable act of terrorism," he said. "But the right response is to finish the work Chris Stevens gave his life to."
Equally, the Prime Minister argued with anyone suggesting that dictatorships had proved the better way for maintaining stability in the region. "We should reject this argument. Brutal dictatorship made the region more dangerous not less," he said.