US President Barack Obama said Tuesday he had both a moral and a national security obligation to stop the slaughter in Syria, but warned he could not just act on a "hope and a prayer."

Obama defended his government's actions in sending large quantities of humanitarian aid to Syrian refugees, providing non-lethal help to rebels and isolating President Bashar al-Assad in the international community.

"I think that understandably, there's a desire for easy answers," Obama said, referring to domestic critics who are demanding a more proactive US role, including an operation to arm rebel groups and a no-fly zone.

"My job is to constantly measure our very real and legitimate humanitarian and national security interests in Syria, but measuring those against my bottom line, which is, what's in the best interests of America's security."

Obama said that he could not make decisions "based on a hope and a prayer, but on hard-headed analysis in terms of what will actually make us safer and stabilize the region."

"I think that we have both a moral obligation and a national security interest in, A, ending the slaughter in Syria but, B, also ensuring that we've got a stable Syria that is representative of all the Syrian people and is not creating chaos for its neighbors," he said.

The president spoke as political critics complain that he has not acted more quickly on reports that Syrian forces had used chemical weapons, an action he had previously said would infringe a red line and be a game-changer.

He said that before acting, Washington must first establish exactly who had used chemical weapons and when, in an apparent reference to the flawed intelligence that led America into war with Iraq.

"I don't make decisions based on perceived," he said, when asked by a reporter about perceived violations of US red lines.

"I can't organize international coalitions around perceived. We've tried that in the past, by the way, and it didn't work out well."

"There have been several instances during the course of my presidency where I said I was going to do something, and it ended up getting done.

"In the end, whether it's bin Laden or Kadhafi, if we say we're taking a position, I would think at this point, the international community has a pretty good sense that we typically follow through on our commitments."