WASHINGTON — 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 a “moral obscenity” that has shocked the world’s conscience.

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

Mr. 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 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 al-Assad, the Syrian president, or to fundamentally alter the nature of the conflict on the ground.