"What we now have is evidence that chemical weapons have been used inside of Syria, but we don't know how they were used, when they were used, who used them. We don't have a chain of custody that establishes what exactly happened," Obama said at a crowded White House news conference.
He said there is no need to rush things.
"And if we end up rushing to judgment without hard, effective evidence, then we can find ourselves in the position where we can't mobilise the international community to support what we do. There may be objections even among some people in the region who are sympathetic with the opposition if we take action," Obama said.
He said the US and its international partners along with the Syrian opposition are trying to gather more information.
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"If I can establish in a way that not only the United States, but also the international community, feels confident about the use of chemical weapons by the Assad regime. That is a game changer because what that portends is potentially even more devastating attacks on civilians and it raises the strong possibility that those chemical weapons can fall into the wrong hands and get disseminated in ways that would threaten US security or the security of our allies," said the US President.
"Now we're already invested in trying to bring back a solution inside of Syria. Obviously, there are options that are available to me that are on the shelf right now, that we have not deployed. And, you know, that's a spectrum of options," he said.
"As early as last year I asked the Pentagon, our military, our intelligence officials to prepare for me what options might be available. And I won't go into the details of what those options might be, but, clearly that would be an escalation of the threat to the security of the international community, our allies, and the US," Obama said.