"I discussed our assessment and (Swedish Prime Minister Fredrik Reinfeldt) and I are in an agreement that in the face of such barbarism the international community cannot be silent," Obama said at a joint news conference in Stockholm along with Reinfeldt.
"Failing to respond to this attack would only increase the risk of more attacks and that possibility that other countries would use these weapons, as well," he said.
Responding to a question, he said the red line against the use of chemical weapons was not set by him.
"I didn't set a red line. The world set a red line. The world set a red line when governments representing 98 per cent of the world's population said the use of chemical weapons are abhorrent and passed a treaty forbidding their use even when countries are engaged in war," he said.
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Meanwhile, Putin in an interview with Russia's state Channel 1 television said any military strikes without UN approval would be "an aggression", as he called for evidence on chemical weapons attack to be presented to the Security Council.
He, however, softened his tone ahead of the G20 summit in Russia, saying he has not ruled out supporting a UN Security Council resolution authorising force, if it was proved "beyond doubt" that the Syrian government used chemical weapons.
"If there is evidence that chemical weapons were used, and by the regular army... Then this evidence must be presented to the UN Security Council. And it must be convincing," Putin said.
But he added that Russia would "be ready to act in the most decisive and serious way" if there was clear proof of what weapons were used and who used them.