The Prime Minister has announced an additional £52 million in aid to Syria in the wake of an alleged chemical attack in Damascus.
Speaking at a G20 meeting he chaired on the humanitarian situation, Mr Cameron said he hoped the leaders present could "send a strong signal that we can act, and act specifically to relieve this appalling suffering caused by the war crime of chemical weapons use".
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.