The Obama administration hardened its stance against Syria and stepped up plans for possible military action, dismissing as too late the regime's offer to let United Nations officials inspect areas where the U.S. believes Damascus used chemical weapons last week.

The White House and Pentagon signaled the U.S. wasn't backing away from a possible showdown despite apparent efforts by the Syrian government to ease tensions by letting U.N. inspectors visit areas near the capital where hundreds were killed, allegedly by chemical...