"We do not seek conflict in Syria, but we cannot allow such grievous violations of international law," Pentagon spokeswoman Dana White told reporters.

"We successfully hit every target," she said. "The strikes were justified, legitimate and proportionate."

Lieutenant General Kenneth McKenzie, also at the briefing, said three sites that are "fundamental components of the regime's chemical weapons infrastructure" were struck.

The operation was "precise, overwhelming and effective," he said, adding it will set their chemical weapons program back "for years."

McKenzie said that none "of our aircraft or missiles involved in this operation were successfully engaged by Syrian air defenses."