Barack Obama said "he was elected to end wars not start them" but added that failing to respond to the chemical weapon attack in Syria would send out the wrong message.
The US President said he would "greatly prefer to work through the United Nations to get this done."
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.