Behind the scenes, the administration’s planning has already shifted to what to do after an expected fall of the Assad government, and what such a collapse could look like. A huge worry, administration officials said, is that in desperation Mr. Assad would use chemical weapons to try to quell the uprising.

“The Syrian government has a responsibility to safeguard its stockpiles of chemical weapons, and the international community will hold accountable any Syrian officials who fails to meet that obligation,” Mr. Carney said.

Any benefit of an Israeli raid on Syria’s weapons facilities would have to be weighed against the possibility that the Assad government would exploit such a raid for its own ends, said Martin S. Indyk, the former United States ambassador to Israel and director of the Foreign Policy Program at the Brookings Institution.

He and several administration officials said the view was that Mr. Assad might use chemical weapons as a last resort. “But it crosses a red line, and changes the whole nature of the discussion,” Mr. Indyk said. “There would be strong, if not overwhelming sentiment, internationally, to stop him.” Russia, in particular, would probably have to drop its opposition to tougher United Nations sanctions against Syria, and Mr. Assad’s other remaining ally, Iran, would probably not look too kindly on a chemical attack.

The Obama administration must also worry about Mr. Assad’s arsenal, including chemical weapons, falling into other hands, including those of Al Qaeda — a risk at the center of the administration’s concerns, according to Robert Malley of the International Crisis Group.

“The government is falling,” Mr. Malley said. “But what will the fall look like? It could fall in Damascus, but not elsewhere; it could crumble in other areas but not the Alawite ones — there are a lot of variations to this.”

Beyond trying to stop the Assad government from using unconventional weapons, the United States must also work to make sure that the Alawite minority, ascendant under Mr. Assad and largely loyal to him, is not massacred once its protector is gone.