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Mr. Kerry said the U.S. will have to act without the U.N.’s blessing because Russia, a key world power supporting the Assad government, would block any resolution approving retaliatory strikes.

“Because of the guaranteed Russian obstructionism of any action through the U.N. Security Council, the U.N. cannot galvanize the world to act, as it should,” Mr. Kerry said.

As a result, he added, “President Obama will ensure that the United States of America makes our own decisions on our own timelines based on our values and our interests.”

The four-page assessment was released to the public, but the administration said it was giving a more detailed assessment to members of Congress that laid out extra information that couldn’t be released because it would compromise sources.

After Mr. Kerry spoke, a senior administration official said the president is “contemplating a military response that is tailored and limited.” But the official said Mr. Obama has not made up his mind.

“The president had not made a final determination about a course of action,” the official said, speaking on condition of anonymity. “He’ll have to make a decision, and he’ll certainly keep the American people informed as he does. It’s important for the president to be prudent and careful in his decision-making.”

Administration officials said there is little doubt that Mr. Assad is ultimately responsible for the attack, saying Syria’s chemical weapons program is “tightly commanded and tightly controlled.”