White House press secretary Jay Carney on Monday reiterated Obama deliberately signaled to Syrian President Bashar Assad that using chemical weapons could provoke a U.S. response, even though that response hasn't been specified.
Carney said it's highly likely that Assad's regime is behind chemical weapons use in Syria, but "we have to be sure about the case that we're making."
"It is very important that we establish conclusively the evidence about chemical weapons use in Syria. And that includes who used it, and under what circumstances, and where," Carney said, noting later "we are highly skeptical of any suggestions, or accusations that the opposition used chemical weapons."
One of the U.N. investigators looking into allegations of human rights originally said Sunday there were strong suspicions that rebel forces used sarin, a nerve agent gas, but the panel later backed off that statement, saying it had not reached "conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
Obama's use of the term "red line" about a year ago was "deliberate" and was based on U.S. policy, Carney said.
"The world knew that the Syrian government possessed chemical weapons and we had a concern that as the regime was increasingly beleaguered, it might use chemical weapons against the Syrian people in desperation," Carney said
When chemical weapons are used, it's a "game-changer," Carney said. "And there are international norms that are violated when chemical weapons are used. It is by definition a game-changer."
Pushing back against criticism for the lack of action by Obama, Carney said the president made clear there was "a red line and that it was unacceptable, and that it would change his calculus as he viewed the situation in Syria because the use of chemical weapons represents the kind of escalation and -- and threat that I just described."
"What he never did, and it is simplistic to do so, is to say that if X happens, Y will happen," Carney said.
"He has never said what reaction he would take at a policy level to the proved crossing of the red line in Syria, simply that he would consider it a red line that had been crossed and that he would take appropriate action."
As the investigation proceeds, Carney said Obama is "looking at a range of options, and he is not removing any option from the table, if you will."