U.S. Secretary of Defense Jim Mattis briefs the press prior to a commemoration of the 70th anniversary of the Marshall Plan at the George C. Marshall Center in Garmisch-Partenkirchen, Germany, Wednesday, June 28, 2017. (AP Photo/Matthias Schrader)

U.S. Secretary of Defense Jim Mattis briefs the press prior to a commemoration of the 70th anniversary of the Marshall Plan at the George C. Marshall Center in Garmisch-Partenkirchen, Germany, Wednesday, June 28, 2017. (AP Photo/Matthias Schrader)

BRUSSELS (AP) — Syria’s government is taking seriously the U.S. warning against launching another chemical weapons attack, Defense Secretary Jim Mattis said Wednesday.

Speaking to reporters traveling with him to Brussels, 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.

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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.”

Trump’s approach has been designed to “dissuade” Syria from using chemical weapons, Mattis added.

A non-governmental source with close ties to the White House said the administration had received intelligence the Syrians were mixing precursor chemicals for a possible sarin gas attack in either the east or south of the country, where government troops and allied forces have faced recent setbacks.

The source was not authorized to discuss the matter publicly, so spoke on condition of anonymity.

Syria and its allies, Russia and Iran, have rejected the U.S. allegation and say Assad’s forces never previously used chemical weapons. The U.S. so far hasn’t provided details to bolster this week’s claim of a chemical attack being planned.

The U.N. has ascribed responsibility to Assad’s government for three previous attacks during Syria’s six-year civil war. In April, Trump ordered almost 60 cruise missiles to be fired at the Shayrat base after accusing Syria of killing dozens of civilians in a sarin gas attack.

Asked if any chemical weapons activity beyond Shayrat has been seen, Mattis told reporters traveling with him to Brussels: “I think that Assad’s chemical program goes far beyond one airfield.”

Mattis is in Brussels for a meeting of NATO defense ministers.