The world must not "contract out" its morality to the United Nations Security Council by giving permanent members such as Russia the final say on whether military action should be taken against Syria to punish Bashar Assad for using chemical weapons against his own people, David Cameron has warned.
Mr Cameron was speaking at the end of an acrimonious summit of the G20 in St Petersburg which exposed stark divisions between host Vladimir Putin and US president Barack Obama, who is seeking Congressional support for punitive strikes.
The Russian president - a close ally of Assad - declared that any attack without a UN resolution would "violate the law". And he indicated that he was ready to give further military assistance to the Damascus regime if it comes under attack from the US and France, saying: "Will we help Syria? We will. And we are already helping - we send arms, we co-operate in the economic sphere."
Amid heightened tensions about the possible use of force, Russia's Interfax news agency reported the country's navy HQ as saying that a fourth Russian naval vessel is on its way to Syria to join three already in the eastern Mediterranean.
Mr Obama was left relatively isolated at the G20 as only France indicated it was ready to join the US in taking military action while Britain, Turkey, Canada and Saudi Arabia voiced support for robust action to respond to the poison gas attack on a Damascus suburb which claimed hundreds of lives on August 21. Mr Obama said there was "a growing recognition that we cannot sit idly by". He announced plans to make an address on Syria from the White House on Tuesday.
But Mr Putin described the use of chemical weapons in Ghouta as a "provocation" by anti-Assad "militants" hoping for support from the outside world, and insisted that military action was opposed not only by the majority of G20 states, but also by a majority of the populations of those countries considering strikes.
In a calculated jab at the Prime Minister, Mr Putin listed those backing military action and added: "Mr Cameron is also in favour, but in Britain Parliament was against that. The application of force against a sovereign state can only be in the case of self-defence. Syria does not represent that. Those who act unilaterally violate the law."
After the most divisive leaders' summit in the event's five-year history, the G20 concluded with a 27-page official communique which did not use the word "Syria" once, concentrating instead on agreements on boosting economic growth and creating jobs.
But 11 members of the group of 20 leading economies - including the UK - issued a joint statement declaring that: "The world cannot wait for endless failed processes that can only lead to increased suffering in Syria and regional instability. We support efforts undertaken by the United States and other countries to reinforce the prohibition on the use of chemical weapons."
This week's summit has witnessed renewed strains on the UK-Russia relationship, with Mr Cameron issuing an impassioned rebuttal of the reported description by a Putin aide of Britain as a "small island" whose voice did not carry weight. Mr Putin's chief spokesman denied making the remark, but Downing Street demanded "clarification".