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

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."

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CBS News' David Martin reports that Mattis's statement that the Syrians appear to have taken the White House warning against another chemical attack seriously is based on the fact that there hasn't been a chemical attack since the warning was issued.

The situation at Shayrat airfield has not changed.

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."