ABOARD A U.S. MILITARY AIRCRAFT — Defense Secretary Jim Mattis said Wednesday that the U.S. warning to Syria not to conduct a chemical attack on its own people appears to have been taken seriously by the Bashar Assad regime.

"They didn't do it," Mattis told reporters on his plane as they flew from Germany to Belgium. "It appears they took the warning seriously."

Mattis would not confirm or discuss on-the-record statements by a Pentagon spokesman that the U.S. had observed the movement of chemical munitions at the Shayrat airfield, the same airfield from which a Syrian warplane dropped deadly sarin nerve gas on civilians April 4. Navy warships launched a cruise missile attack on the air base later that week, which destroyed several aircraft.

But Mattis did say he believed that Syria has retained chemical weapons in other locations in the country in violation of an agreement to give up all its chemical stockpiles.

"I think that Assad's chemical program goes far beyond one airfield," Mattis said.

Asked about his level of confidence that Assad has backed off, Mattis said, "I'm not paid to have confidence in this sort of thing. I'm paid to be one of the sentinels that watches for it."

The White House on Monday night took the unusual step of warning the Assad regime ahead of a possible attack.

"The United States has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children," the White House statement said. "The activities are similar to preparations the regime made before its April 4, 2017, chemical weapons attack.

"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria. If, however, Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."

On Tuesday, Navy Capt. Jeff Davis, a spokesman at the Pentagon, explained what drove the announcement.

"We had seen activity at Shayrat airfield, the same airfield we struck in April, that indicated active preparations for chemical weapons use," Davis said.

Mattis met with his counterpart in Germany on Tuesday before heading to Brussels for a meeting of NATO defense ministers to discuss the plan for Afghanistan.