WASHINGTON — Moving a step closer to possible American military action in Syria, a senior Obama administration official said Sunday that there was “very little doubt” that President Bashar al-Assad’s military forces had used chemical weapons against civilians last week and that a Syrian promise to allow United Nations inspectors access to the site was “too late to be credible.”

The official, in a written statement, said that “based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts and other facts gathered by open sources, the U.S. intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident.”

The statement, released Sunday morning on the condition that the official not be named, reflected a tougher tone after President Obama’s meeting at the White House on Saturday with his national security team, during which advisers discussed options for military action.

While administration officials emphasized that Mr. Obama had not decided to take action, they said he was determined not to be drawn into a protracted debate over gaining access for the United Nations investigators, because of doubts that they could now produce credible findings.