"In terms of what my goals are, the goals are a stable, non-sectarian, representative Syrian government that is addressing the needs of its people through political processes and peaceful processes," Obama said.
"We're not taking sides in a religious war between Shia and Sunni," he said.
"Really, what we're trying to do is take sides against extremists of all sorts and in favour of people who are in favour of moderation, tolerance, representative government, and over the long-term, stability and prosperity for the people of Syria," he said in an interview to PBS channel.
"We've been assisting not only the political opposition but also the military opposition. So, there's a counter-weight that can potentially lead to political negotiations -- with the evidence of chemical weapons, what we've said is we're going to ramp up that assistance," Obama said.
"And my hope continues to be, however, that we resolve this through some sort of political transition," he said.
"But what's been clear is that Assad, at this point -- in part, because of his support from Iran and from Russia -- believes that he does not have to engage in a political transition, believes that he can continue to simply violently suppress over half of the population," the US President said.
The US, he said, is ramping up support for both the political and military opposition.
"I've said I'm ramping up support for both the political and military opposition. I've not specified exactly what we're doing and I won't do so on this show. That's point number one," he said.
"Point number two is that this argument that somehow we had gone in earlier, or heavier in some fashion, that the tragedy and chaos taking place in Syria wouldn't be taking place, I think is wrong," Obama said.
"These aren't professional fighters," he added.