The White House said late Monday that President Bashar al-Assad of Syria appeared to be preparing another chemical weapons attack, and warned that he would “pay a heavy price” if one took place.
Several military officials were caught off guard by the statement from President Trump’s press secretary, but it was unclear how closely held the intelligence regarding a potential chemical attack was.
In the statement, the White House said that Mr. Assad’s preparations appeared similar to the ones Western intelligence officials believe the Syrian government made before a chemical attack in April that killed dozens of Syrians, including children.
“As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria,” the statement said. “If, however, Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price.”
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 targetany 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 “in at least the last 20 years.” More often, such matters are handled in private diplomatic or intelligence communications, he said.
Monday’s 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 Obama’s intelligence agencies concluded with “high confidence” 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 “red line” 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. Assad’s cache of chemical weapons. Critics argued that the president’s failure to enforce his own “red line” had emboldened Mr. Assad.
They also warned that all of the chemical weapons could not be found and destroyed. Within two months of Mr. Trump’s taking office, images of another chemical attack spurred him to take the action that Mr. Obama had rejected.