President Obama said the US wanted to "join with the international community in an effective response" that deters the use of chemical weapons in the future.
He said: "I respect the UN process. Obviously the UN investigation team has done heroic work under very difficult circumstances but we believe very strongly, with high confidence, that in fact chemical weapons were used and that Mr Assad was the source.
"We want to join with the international community in an effective response that deters such use in the future."
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.