The Prime Minister used his moment of fame at the United Nations to criticise those countries which had, as he put it, aided and abetted President Assad's reign of terror in Syria.
Though he didn't say it, the message was directed at Russia and China who have blocked more strident action against in the Security Council.
Outside the UN's headquarters in New York, there were protests against the organisation's apparent impotence in the face of violence.
ITV News' Washington Correspondent Robert Moore reports from New York:
The hands of the UN are stained with blood due to its failure to stop the Assad regime in Syria, David Cameron told the UN General Assembly.
David Cameron will urge the world to give support to the emerging democracies of the Arab Spring as he addresses the UN General Assembly.
President Obama said today that the US would not tolerate a nuclear-armed Iran, but that won't extinguish the heated Israeli rhetoric.