Michael R. Gordon and Mark Landler / New York Times News Service

Secretary of State John Kerry said Monday that the use of chemical weapons in attacks on civilians in Syria last week was undeniable and that the Obama administration would hold the Syrian government accountable for what he called a “moral obscenity” that had shocked the world’s conscience.

In some of the most aggressive language used yet by the administration, Kerry accused the Syrian government of the “indiscriminate slaughter of civilians” and of cynical efforts to cover up its responsibility for what he called a “cowardly crime.”

Kerry’s remarks at the State Department reinforced the administration’s toughening stance on the Syria conflict, which is now well into its third year, and indicated that the White House was moving closer to a military response in consultation with America’s allies.

Administration officials said that although President Barack Obama had not made a final decision on military action, he was likely to order a limited military operation — cruise missiles launched from American destroyers in the Mediterranean Sea at military targets in Syria, for example — and not a sustained air campaign intended to topple Bashar Assad, the Syrian president, or to fundamentally alter the nature of the conflict on the ground.

In the coming days, officials said, the nation’s intelligence agencies will disclose information to bolster their case that chemical weapons were used by Assad’s forces.

Officials said it was conceivable that military action could still be averted by a dramatic turnabout on the part of the Assad government, or by the Russian government that has been supporting it. But they said there were few expectations that this would happen.

Although the United States was consulting with allies, administration officials said they had largely abandoned hopes of obtaining any authorization for action in the U.N. Security Council, given the all-but certain veto from Russia.

Other signs of Western momentum toward a military response also took shape on Monday. Britain’s prime minister, David Cameron, cut short a vacation to deal with Syria, and the foreign ministers of Britain and Turkey suggested that bypassing the U.N. Security Council was an option.

U.N. investigation — U.N. experts collected samples and testimony from Syrian doctors and victims of an alleged chemical weapons attack Monday following a treacherous journey through government and rebel-held territory, where their convoy was hit by snipers.

As U.S. officials said there was very little doubt that Syria used chemical weapons and Western powers stepped up calls for swift military action, President Bashar Assad’s government vowed to defend itself against any international attack, warning that such an intervention would ignite turmoil across the region.

It also would bring the U.S. closer to a conflict that has killed more than 100,000 people since Assad cracked down on Arab Spring-inspired protesters in March 2011.

Syria’s civil war has been increasingly defined by sectarian killings between the Sunni-led rebellion and Assad’s regime, dominated by Alawites, an offshoot of Shiite Islam.

— The Washington Post