David Cameron said because there was never going to be an agreement on military action at the G20 summit, Britain decided to focus its effort on securing more humanitarian aid for the crisis in Syria.
He said the the case made by those countries who believed there was a need to make a stand was "extremely powerful."
He reiterated that Britain will not be part of any military action.
The G20 summit in St Petersburg exposed stark divisions between host Vladimir Putin and US President Obama over Syria.
War emptied the suburb of Yarmouk in southern Damascus of its people. Assad's army holds it, but their prize is a ghost town.
Even in the President's heartland of Baltimore, 95% of voters are against his policy on military intervention in Syria.