"This is going to be a long-term proposition. This is not going to be something that is solved easily overnight," Obama said.
For months, the administration has warned Syrian President Bashar Assad not to use chemical weapons in his struggle against opposition fighters. Obama has talked tough about what could trigger a U.S. response with "enormous consequences," but he has never spelled out what those consequences might be.
White House press secretary Jay Carney reiterated Friday that Obama "retains all options to respond" if further information proves the suspected use of the nerve agent sarin.
The definitive proof the White House is seeking is likely to be weeks or months in the offing, if it comes at all. A U.N. weapons team has been blocked from on-the-ground testing, and it is not clear what other scientific or intelligence information the White House would find persuasive.
Analysts say the White House's statement this week that it wants further proof from the United Nations or other investigators effectively pushes off the need for a decision.
"This is what he's done all along the line," Joshua Landis, a Syria expert at the University of Oklahoma, said of Obama. "He punted to the U.N., where he knows it will die a soft and ignominious death."
Last month, U.N. Secretary General Ban Ki-moon appointed a 15-person team to inspect sites in Syria where chemical weapons might have been used. But the team has been cooling its heels because Assad refuses to grant access to all of the locations, and U.N. diplomats see no end in sight to the stalemate.
Efforts to take a tougher line on Syria by the United States and its allies at the United Nations have been blocked by Russia, which remains an Assad ally.
The United States is under pressure to do more from several Arab states, along with Turkey, Israel and prominent European allies. There is no consensus about what those nations want from Obama, but there is little pressure from the American public for military involvement.
The White House followed allies Britain, France and Israel in concluding that chemical weapons were probably used by the Syrian regime over the past four months, but it added the caveat that the assessment was reached "with varying degrees of confidence."
"We're going to be working with countries like Jordan to try to obtain more direct evidence and confirmation of this potential use," Obama said Friday. "In the meantime, I've been very clear publicly but also privately that for the Syrian government to utilize chemical weapons on its people crosses a line that will change my calculus and how the United States approaches these issues."
He avoids, for now, the question of what he would do if Assad ignored his ultimatum.
Responses could range from the purely rhetorical and diplomatic to precision airstrikes on Syrian air defenses and chemical stockpiles. Analysts say there is little likelihood of a major U.S. military presence in Syria under any circumstances.
Obama already faces the prospect of a military confrontation with Iran over its nuclear program after vowing that he would use all means necessary to prevent Tehran from developing a nuclear bomb.
Any action in Syria must be weighed against his policy toward an Iranian government on the lookout for signs of faltering American resolve. A U.S. military intervention in Syria would open up a new front in the Islamic world but could also serve notice to Iran that Obama means what he says when he draws red lines.