"If you make a flat statement like that and you don't follow it up, then you undermine your credibility," says Lawrence Korb, a former Pentagon official who is now a national security analyst at the Center for American Progress (CAP) in Washington.
Obama has been reluctant to deepen US involvement in Syria's war, limiting US assistance to food and supplies for refugees and internally displaced Syrian civilians, and to nonlethal material for the rebel fighters the US supports. But use of chemical weapons by Mr. Assad's forces could prompt a more interventionist approach, some analysts say: for example, direct measures by US forces to destroy or safeguard Assad's chemical weapons. Obama could also cite a crossed red line as justification for arming the rebels or taking other, more robust measures to protect Syrian civilians.
Such measures might include establishing a no-fly zone over northern Syria, a step that already has bipartisan support in Congress, or creating "humanitarian corridors" for refugees to move along and for getting food and other supplies to the civilian population.
Probably a last resort, Mr. Korb says, would be "to send in special forces to grab the chemical weapons."
Pentagon officials said last month they were preparing a list of calibrated measures that the president could order in the event that chemical weapons are used. In response to the Israeli claims Tuesday, Pentagon spokesman George Little said the US "continues to assess reports of chemical weapons use in Syria."