The White House warned on Tuesday that the United States and its allies would respond "swiftly and appropriately" if Syria's Bashar Assad used chemical weapons again.
The White House also said in a statement that it was closely monitoring developments in the opposition-held Idlib region, where Syrian regime forces are expected to mount an offensive that could spark a humanitarian disaster.
The White House says Trump has warned that such an attack would be a "reckless escalation of an already tragic conflict" and would put at risk the lives of hundreds of thousands of people.
Russia has been a key supporter of Assad and complained about militants in Idlib targeting its own facilities.
