The Prime Minister was speaking at the end of an acrimonious summit of the G20 in St Petersburg, which exposed stark divisions between Russian host Vladimir Putin and US president Barack Obama, who is seeking support for air strikes to punish Bashar Assad for using chemical weapons against his own people.
The Russian president, a close ally of Assad, declared that any attack without a UN resolution would "violate the law".
He also said he was ready to give further military assistance to the Syrian regime if it comes under attack from the US and France, saying: "Will we help Syria? We will. We are already helping. We send arms and we cooperate in the economic sphere."
Amid heightened tensions over the issue of force, Russia's Interfax news agency reported the country's navy HQ as saying that a fourth Russian 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 strikes, while the UK, Turkey, Canada and Saudi Arabia voiced support for robust action to respond to the gas attack on a Damascus suburb that claimed hundreds of lives on August 21.
The US president insisted he was "encouraged by my discussions with world leaders this week" and 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.
However, Mr Putin described the chemical weapons attack 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 most citizens of countries considering strikes.
"Mr Cameron is also in favour," Mr Putin added, "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."
Mr Cameron made clear he does not believe that military action must wait until UN inspectors report on their investigations at the scene of the outrage, pointing out that their remit does not extend to apportioning blame.
Scientists, including UK experts at Porton Down, have already found positive evidence of nerve gas sarin, building up an "overwhelming picture of a war crime on August 21", he said.
In a swipe at Mr Putin, Mr Cameron said it was "impossible to believe" that the opposition were responsible for gassing one of their own strongholds with weapons they do not possess. "In spite of that," he said, "some people, including people at this G20 conference, take that view."
The summit has led to a renewed strain on UK-Russia relations, 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".