The possible shift, first reported by The Washington Post on Tuesday, came days after the United States disclosed its preliminary intelligence assessment that chemical munitions, which Mr. Assad’s military has been known to stockpile, had been used on a small scale in Syria. Mr. Obama, who has said the use of these weapons would be a “game changer,” has also said that before responding, he needs more conclusive proof that Mr. Assad had deployed these weapons — a point he re-emphasized at a wide-ranging news conference in Washington earlier Tuesday.

“What we now have is evidence that chemical weapons have been used inside of Syria, but we don’t know how they were used, when they were used, who used them; we don’t have chain of custody that establishes what exactly happened,” Mr. Obama said. “And when I am making decisions about America’s national security and the potential for taking additional action in response to chemical weapon use, I’ve got to make sure I’ve got the facts.”

Mr. Obama also said that “if we end up rushing to judgment without hard, effective evidence, then we can find ourselves in the position where we can’t mobilize the international community to support what we do.”

But even without conclusive proof of chemical weapons use, officials said, the Pentagon has prepared a menu of options for Mr. Obama that range from airstrikes and commando raids to the enforcement of a no-fly zone over Syria. Officials said the administration was also looking for ways to increase its aid to the rebels.

Mr. Obama had long resisted calls to arm the rebels, including from David H. Petraeus, the former director of the Central Intelligence Agency, and former Secretary of State Hillary Rodham Clinton. A proposal by Mr. Petraeus to provide arms to carefully vetted members of the opposition was shelved last fall, though several officials said they expected it to be revisited.