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.
Mattis wouldn't say what specifically triggered US concerns that an attack might be imminent.
He warned against any unilateral action, violation of global law as well as the sovereignty of Syria, and said, "It goes without saying that any adventurism and unjustified action by United States in Syria amounts to playing with fire".
Trump's approach has been created to "dissuade" Syria from using chemical weapons, Mattis added.
The source was not authorized to discuss the matter publicly, so spoke on condition of anonymity.
Though the IS group is on the retreat, it still holds approximately 250 kilometers (155 miles) of territory along the Euphrates River in Syria, which has come under enormous pressure as rival USA -backed Syrian Kurdish forces and Russian- and Iranian-backed Syrian government forces race for control of the resource-rich region.
"If the USA administration is honest in its claims it must put the related information at the disposal of the Organization for the Prohibition of Chemical Weapons so that the issue is verified independently through cooperation with the Syrian government".
Russian Federation is Assad's key backer and sided with him when he denied responsibility for a chemical weapons attack that killed dozens of people in Idlib province on April 4. In April, Trump ordered nearly 60 cruise missiles to be fired at the Shayrat base after accusing Syria of killing dozens of civilians in a sarin gas attack.
At the same time, he cautioned that "Assad's chemical programme goes far beyond one airfield".
Mattis is in Brussels for a meeting of North Atlantic Treaty Organisation defense ministers.
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