One senior administration official disclosed that the United States was examining whether Mr. Assad’s actions constituted war crimes and whether it was possible to seek international legal action against him, his government or Syria’s police forces and military.

The official said the United States was “looking into” whether “there are grounds here for charges related to war crimes, and whether referrals on that are appropriate.”

He warned that actions by the government in Damascus were “headed toward making Syria a pariah state around the world.”