President Obama is likely to order a limited airstrike on Syria to punish it for using nerve gas but won't seek to topple the Assad regime, U.S. officials said.
The strike, whose timing was left open but described as all but inevitable, would likely be conducted with Britain and perhaps other allies, and would probably last only a day or two, officials said.
It would probably involve long-range guided cruise missiles launched from sea or from bomber aircraft, the officials said.
Missile-armed U.S. warships are already positioned in the Mediterranean Sea. A British nuclear-powered submarine is also in the Mediterranean and several British warships are on their way.
British military aircraft are building up at a large Royal Air Force base on the Mediterranean island of Cyprus, British newspaper The Guardian reported.
Cyprus is about 200 miles due west of Syria.
The probable U.S.-British-led attack, which might have French, German and other allied support, would be characterized by extreme precision and incisiveness to avoid bringing the United States into deeper involvement in Syria's civil war, Obama administration officials told several news organizations.
The missiles would strike military targets not directly related to Syria's chemical weapons storage areas, officials told The Washington Post.
Those arsenals areas are numerous and widely dispersed.
They would also not seek to cripple President Bashar Assad's military or change the balance of forces on the ground, the officials told the Post and The New York Times.
But at the same time, the Assad regime will be punished for the "undeniable" use of chemical weapons in Wednesday's attack against opposition strongholds on the eastern outskirts of Damascus, Secretary of State John Kerry said at the State Department Monday.
He called the "indiscriminate slaughter of civilians" and the "cynical attempt" by the Assad regime to cover up its responsibility a "cowardly crime."
"Make no mistake," Kerry said. "President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people. Nothing today is more serious and nothing is receiving more serious scrutiny."
Evidence gathered by U.N. weapons inspectors in Syria was important but not needed to prove what is already "grounded in facts, informed by conscience and guided by common sense," Kerry said.
"Anyone who can claim that an attack of this staggering scale could be contrived or fabricated needs to check their conscience and their own moral compass," he said.
Assad told Russian newspaper Izvestia in an interview published Monday accusations his forces used chemical weapons were an "outrage against common sense."
He warned U.S. officials a military intervention in Syria would bring "failure just like in all the previous wars they waged, starting with Vietnam and up to the present day."
The Assad regime and Russia have suggested opposition rebels were responsible for the chemical attack -- an argument Kerry said was impossible to take seriously.
Russian President Vladimir Putin told British Prime Minister David Cameron late Monday he saw no evidence a chemical attack had taken place, much less that the Assad regime was responsible, a British spokesman said, describing the phone call.
Both leaders agreed with a position the Group of Eight took at its June summit in Northern Ireland that "no one should use chemical weapons and any use would merit a serious response from the international community," the British spokesman said.
Russian Foreign Minister Sergei Lavrov warned Monday any attack on Syria without U.N. Security Council approval would be a "grave violation of international law."
His comments came as two mortars landed near the Four Seasons hotel where U.N. weapons inspectors are staying before they set off to examine the alleged chemical attacks in the suburban Ghouta region east of Damascus. Then, on the way to the area, the inspectors' convoy was hit by unidentified sniper fire as it crossed the buffer zone from the regime-controlled center of Damascus to the rebel-held area east of the city.
The inspectors managed to visit two hospitals, interview witnesses and doctors, and collect patient samples for the first time since the attack the opposition claims killed more than 1,100 people.
There was no indication any member of the inspection team was hurt by the sniper fire.