WASHINGTON — 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.”