While the White House’s motivation in releasing the highly unusual statement is uncertain, it is possible that Mr. Trump or his advisers decided a public warning to Mr. Assad might deter another chemical strike.
Any intelligence gathered by the United States or its allies — notably Israel, which keeps a robust watch on unconventional weapons in the Middle East — would by nature be classified. But any American president has absolute power to declassify anything he chooses to release.
Brian Hale, a spokesman for the director of national intelligence, referred questions to the White House. Marc Raimondi, a spokesman for the White House’s National Security Council, said, “We are letting the statement speak for itself.”
Nikki R. Haley, the American ambassador to the United Nations, made clear that the United States was taking the latest threat seriously. “Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia & Iran who support him killing his own people,” she tweeted late Monday.
Russia and Iran are both allied with the Assad government. Last week, after the United States downed a Syrian warplane that had dropped bombs near American-supported fighters battling the Islamic State, Russia’s Defense Ministry threatened to target any aircraft flown by the United States or its allies west of the Euphrates River valley.
Such a threat can cause an unintended showdown as competing forces converge on ungoverned areas of Syria. The collision has effectively created a war within a war.
Daryl G. Kimball, the executive director of the Arms Control Association, said that he had not heard of Syrian moves toward more chemical attacks, but that he suspected intelligence reports had prompted the statement. Rocket attacks using sarin gas, as in the April strikes, require considerable preparation that American intelligence might well have picked up, he said.
Mr. Kimball added that he did…