WASHINGTON —

With the growing conviction that the Assad family's 42-year grip on power in Syria is coming to an end, Obama administration officials worked on contingency plans Wednesday for a collapse of the Syrian government, focusing particularly on the chemical weapons that Syria is believed to possess and that President Bashar Assad could try to use on opposition forces and civilians.

Pentagon officials were in talks with Israeli defense officials about whether Israel might move to destroy Syrian weapons facilities, two administration officials said. The administration is not advocating such an attack, U.S. officials said, because of the risk that it would give Assad an opportunity to rally support against Israeli interference.

President Obama's national-security adviser, Thomas Donilon, was in Israel over the weekend and discussed the Syrian crisis with officials there, an administration official said.

Obama called Russian President Vladimir Putin on Wednesday and urged him again to allow Assad to be pushed from power. Russia has refused. An administration statement said Putin and Obama "noted the growing violence in Syria and agreed on the need to support a political transition as soon as possible that achieves our shared goal of ending the violence and avoiding a further deterioration of the situation."

The statement noted the "differences our governments have had on Syria," but said the two leaders "agreed to have their teams continue to work toward a solution."

U.S. diplomatic and military officials said the bombing in Damascus on Wednesday that killed several of Assad's closest advisers was a turning point in the conflict. "Assad is a spent force in terms of history," Obama spokesman Jay Carney said. "He will not be a part of Syria's future."

Alluding to Russia's position, Carney said the argument that Assad's ouster would result in more violence was contradicted by the bombing, and Assad's continued rule "will result in greater violence," not less.

Within hours of the bombing, the Treasury Department announced additional sanctions against the Syrian prime minister and some 28 other Cabinet ministers and senior officials, part of the administration's effort to make life so difficult for the government that Assad's allies desert him. "As long as Assad stays in power, the bloodshed and instability in Syria will only mount," said David Cohen, a senior Treasury official.

Behind the scenes, the administration's planning has 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 Assad in desperation 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 fail to meet that obligation," 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 ends, said Martin Indyk, a former U.S. ambassador to Israel and director of the Foreign Policy Program at the Brookings Institution.

He and several administration officials said the view was that Assad might use chemical weapons as a last resort. "But it crosses a red line, and changes the whole nature of the discussion," Indyk said. "There would be strong, if not overwhelming sentiment, internationally, to stop him."

Russia, in particular, probably would have to drop its opposition to tougher U.N. sanctions against Syria, and Assad's other remaining ally, Iran, probably would not look too kindly on a chemical attack.

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

"The government is falling," 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 U.S. also must work to make sure the Alawite minority, ascendant under Assad and largely loyal to him, is not massacred once he is gone.

Obama has come under criticism from some congressional Republicans who say the United States should intervene militarily in Syria, and from Mitt Romney, his Republican opponent, who has said he would arm the Syrian opposition, which the administration has not done directly.

Instead, Obama has backed U.N. efforts and urged Russia to join the United States in calling for Assad to step down. While the president has been faulted for his policy toward Syria, some foreign-policy experts said Obama's approach could be vindicated, particularly if Assad is toppled without U.S. military action.

The administration has not armed the Syrian rebels officially, but it has provided financial aid and has helped to prop up the Syrian opposition by its efforts to delegitimize Assad through a stream of calls for him to step down.

The U.S., Malley said, "may actually achieve what it wanted: a fall of the regime without having to intervene militarily."

But, he added, "Then it has to deal with all the variants of what a fall looks like, and what a post-Assad Syria looks like."

New York Times writer Peter Baker contributed to this report.