G20: Syria policy cannot be left to United Nations says Cameron
Relying on the UN to act over Syria would be tantamount to "contracting out foreign policy and morality" to a Russian veto, David Cameron has said.
Speaking at the G20 summit in St Petersburg, the prime minister said the international community could not be hamstrung by a likely Russian veto.
Countries supporting intervention had "far the better of the argument" in discussions, Mr Cameron added.
But Vladimir Putin said more than half were opposed to any military action.
Little progress was made on Syria at the two-day summit, although the UK said agreement had been reached on increasing access for humanitarian aid and medical assistance.
The prime minister said he continued to press for a tough response to the "taboo" of chemical weapons - adding that the case they had been used by the Assad government was "overwhelming".
'Taking a stand'
He said he backed US President Barack Obama's view that this was a "red line" and, although the UK will not take part in any military action following last week's vote in Parliament, he was "convinced" that the US would be able to build a coalition for action.
"This summit was never going to reach agreement on what action is needed on Syria," he told reporters.
"But the case made by those countries who believe we have to take a stand against the use of chemical weapons was, I believe, extremely powerful.
"President Obama has set out that case again at this summit and I will support his efforts to build international support for upholding the international taboo against the use of chemical weapons."
Mr Cameron said he disagreed with President Putin's view that military intervention could only ever be justified in self-defence or if authorised by the UN Security Council.
"I think we do have, internationally, to take on this argument.
"If we accept that the only way a response can be made to a country that, let's say, was massacring more than half its people, if we are saying there can only be a response if the UN security council votes positively we are in fact contracting out our foreign policy, our morality, to the potential of a Russian veto.
"I think that is a very misguided approach."
Ten countries, including the UK, US, France, Turkey and Saudi Arabia, have signed an agreement condemning the actions of the Assad government and stating there must be repercussions.
The US President said he was "not itching" to launch a military strike.
He said he was only contemplating such a move because of the gravity of the crime committed and because he was, at this stage, not persuaded that any other course of action would be effective in deterring the future use of such weapons.
But the Russian president said countries such as Brazil and India were opposed to military action, that there were major reservations across Europe and public opinion was against such a move in the US and France.
On economic matters, Mr Cameron said Britain was helping to set the agenda for global economic recovery and all 20 countries had signed up to an action plan which mirrored the policies the coalition government had been following since it came to power.
These included the importance of dealing with debt, the role of monetary policy to support the recovery and the need for long-term reforms to boost growth and trade and cut red tape, the prime minister said.
He added that the summit had also adopted what he called the "bold pro-business agenda" set at June's G8 meeting in Northern Ireland, including plans for tax authorities across the world to automatically share information to prevent tax evasion.