BRUSSELS (AP) — Defense Secretary Jim Mattis says Syria's government has taken seriously the U.S. warning against launching another chemical weapons attack.

Mattis noted there has been no such attack since the White House issued a surprise statement Monday night that threatened President Bashar Assad's government with "a heavy price" if it used chemical weapons.

The U.S. says it saw active preparations at Syria's Shayrat airfield for using such weapons. Mattis wouldn't say what specifically triggered U.S. concerns that an attack might be imminent. He said President Donald Trump has showed "how seriously we took them."

Asked if other activity has been seen, Mattis told reporters traveling with him to Brussels: "I think that Assad's chemical program goes far beyond one airfield."