While the White Houseâs 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 â notably Israel, which keeps a robust watch on unconventional weapons in the Middle East â 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 Houseâs National Security Council, said, âWe are letting the statement speak for itself.â
Nikki R. Haley, the American ambassador to the United Nations, made clear that the United States was taking the latest threat seriously. âAny 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,â 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, Russiaâs 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 not recall such a precise, pre-emptive public warning against a foreign government regarding banned weapons âin at least the last 20 years.â More often, such matters are handled in private diplomatic or intelligence communications, he said.
Mondayâs message appeared designed to set the stage for another possible military strike. After Mr. Assad allegedly used chemical weapons in April, the American military fired 59 Tomahawk cruise missiles at the air base his government had used to launch the attack.
The use of chemical weapons by the Syrian government has long been part of the clash between Mr. Assad and the United States.
In 2013, President Barack Obamaâs intelligence agencies concluded with âhigh confidenceâ that Mr. Assad had carried out a devastating chemical attack that killed hundreds of Syrians, despite having been warned by Mr. Obama against crossing a âred lineâ by using chemical weapons.
But Mr. Obama stopped just short of ordering a military strike, instead opting to work with the Russian government to identify and destroy Mr. Assadâs cache of chemical weapons. Critics argued that the presidentâs failure to enforce his own âred lineâ had emboldened Mr. Assad.
They also warned that all of the chemical weapons could not be found and destroyed. Within two months of Mr. Trumpâs taking office, images of another chemical attack spurred him to take the action that Mr. Obama had rejected.