“Read for yourselves the evidence from thousands of sources,” Mr. Kerry said in aggressively laying out the administration's case for a strike on Syria. “This is the indiscriminate, inconceivable horror of chemical weapons. This is what Assad did to his own people.”

Mr. Kerry said that more than 1,400 people were killed in the chemical attack, including more than 400 children.

A four-page intelligence summary released by the White House said the government had concluded that the Assad government had “carried out a chemical weapons attack” outside Damascus, based on human sources as well as communications intercepts. The suggestion that the opposition might have been responsible “is highly unlikely,” the assessment said.

The president said he was continuing to consult with Congress and allies in other countries, but said that any attack would not involve American troops on the ground in Syria.

“We're not considering any open-ended commitment. We're not considering any boots on the ground approach,” Mr. Obama told reporters before meeting with Baltic leaders in the White House. He said he had “not made any decisions” about what actions the United States would take in Syria.

Mr. Kerry said the administration had “high confidence” in the intelligence, much of which was being released to the public as he spoke. But he vowed that the government had carefully reviewed the evidence to avoid the kind of intelligence failures that preceded the Iraq war.

“We will not repeat that moment,” he said.

Mr. Kerry said the time for questions about what happened in Syria had passed.

“The question is whether we — we collectively — what are we and the world going to do about it?” Mr. Kerry said. He said that taking action in the face of the use of chemical weapons “matters deeply to the credibility and the future interests of the United States.”

Mr. Obama said that many people were “war weary,” and — without singling out the decision by Britain not to join in any attack — he added that “a lot of people think something should be done, but nobody wants to do it.”

The president said he appreciated that there was a “certain weariness” following the war in Afghanistan and a suspicion about military action in the aftermath of the Iraq war. But he said that the United States would be sending a message to the world if it did nothing.

“It's important for us to recognize that when over a thousand people are killed, including hundreds of innocent children, through the use of a weapon that 98 or 99 percent of humanity says should not be used even in war, and there is no action, then we're sending a signal,” he said.

American intelligence agencies in the three days before the Aug. 21 attack detected signs of activities by the Syrian authorities “associated with preparations for a chemical weapons attack,” the assessment said. Syrian chemical weapons personnel were operating in the suburb of Adra from Aug. 18 until early on the morning of Aug. 21. On that date, it added, a “Syrian regime element prepared for a chemical weapons attack,” including the use of gas masks.

Spy satellites detected rocket launchings from government-controlled territory 90 minutes before the first reports of a chemical weapons attack. The intelligence agencies said they had identified more than 100 videos related to the attack, many showing large numbers of bodies with physical signs consistent with nerve agents, and they added that the Syrian opposition “does not have the capability to fabricate all of the videos.”

The agencies also said they had intercepted the communications of a senior Syrian official who “confirmed that chemical weapons were used by the regime on Aug. 21 and was concerned with the U.N. inspectors obtaining evidence,” the assessment said. It added that on the afternoon of that day, Syrian chemical weapons personnel were directed to cease operations.

The administration has repeatedly said there is no question that the government of Mr. Assad used chemical weapons against its own people in an attack that killed hundreds of people.

That would cross the red line that Mr. Obama drew last year, when he declared that the large-scale use of chemical weapons by Mr. Assad would “change my calculus” about American involvement in Syria's bloody civil war.

Pentagon officials have moved warships and other military assets closer to Syria in preparation for a possible attack, which would most likely involve the use of cruise missiles. Defense Secretary Chuck Hagel has said the military is ready to execute any decision by Mr. Obama.

New York Times