Putin and Obama
Russian president Vladimir Putin (L) and US president Barack Obama at the G20 (Reuters)

US president Barack Obama said he was elected to end wars, not start them as he prepares to address the American people from the White House to make a case of military action in Syria.

Talking at the end of the two-day G20 summit in St Petersburg, Obama said that "there's a growing recognition that the world can't just stand by" in the face of use of chemical weapons and failing to respond to the attack "would send a wrong signal to rogue nations, authoritarian regimes and terrorist organisations."

"Syria's Assad brazen use of chemical weapons poses a threat to global peace and security," he said, adding that he and other leaders had had a "full airing of views on the issue".

"The majority of the room is comfortable with our conclusion that the Assad government was responsible for the use of chemical weapons," Obama said, referring to the G20 nations. "Of course that is disputed by President Putin."

But given the paralysis at the UN Security Council "if we are serious that international response is required that will not come through the Security Council action," the US president said.

"I would prefer working through multilateral channel and UN to get this done," he said. "But also I strongly believe that when there is a breach of a norm that is important and the international community is paralysed and does not act, then that norm unravels and then other norms start unravelling. "1400 people were gassed, over 400 of them were children. I was elected to end wars not start them."

Obama also held a surprise meeting with Putin and while they disagreed, the meeting was "candid and constructive."

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