Taking the podium at the United Nations yesterday, David Cameron accused two of the world's leading powers, China and Russia, of blocking international efforts to halt the war in Syria and suggested they had "aided and abetted" President Assad's "reign of terror".
While Mr Cameron did not mention them by name, it was clear he was alluding to Russia and China and their unwillingness on three occasions to approve resolutions in the Security Council to pressure Assad to step down.
Perhaps more striking, however, was the allegation that they have been assisting Assad's war on the rebels and the Syrian people.
Mr Cameron did not specify what kind of assistance he was referring to, whether mere moral support or the delivery of arms. He prefaced the attack by quoting the words of a 16-year-old Syrian called Wael, who was detained in a police station in 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 blood of these children is a terrible stain on the reputation of this United Nations," the Prime Minister said. "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."
The grave tone of Mr Cameron in the cavernous General Assembly hall was in stark contrast with an appearance planned for later last night on The Late Show with David Letterman on the CBS network, before his departure to Brazil for a two-day official visit.
Mr Cameron last night also offered new assistance to Egypt to reclaim assets stolen by the former regime of Hosni Mubarak, with the creation of a British task force which will travel to Cairo and assist in the unravelling of European Union regulations to make the unfreezing of the monies possible.
The Prime Minister also paused to pay tribute to the American 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. "Brutal dictatorship made the region more dangerous not less," he said.
He also urged patience when countries elect Islamist governments. He said the test of such governments lay in what they did once in power.
"Islam is a great religion observed peacefully and devoutly by over a billion people. Islamist extremism is a warped political ideology supported by a minority," Mr Cameron said.