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 what he called a “moral obscenity” that had shocked the world’s conscience. In some of the most strident language used yet by the administration, Mr. Kerry accused the Syrian government of cynically seeking to cover up the use of the weapons, and he rejected its denial of responsibility for what he called a “cowardly crime.”