US Defence Secretary Chuck Hagel told a House of Representatives hearing the "likelihood is very high" that Syrian President Bashar al-Assad would use chemical weapons again if America fails to act.
The US defence chief told the House Foreign Affairs Committee that a strike on Syria would not be a "pin prick" and would reduce Assad's military capability.
Obama made a personal appeal to the international community to unite in military action against 'barbarism' in Syria.
President Obama has received the support of prominent Republicans as he tries to gain backing for military action in Syria.
President Obama and his team are engaged in frenzied lobbying as lawmakers prepare to vote on a possible military strike in Syria.