"We have seen activity at Shayrat airfield... That indicated preparations for possible use of chemical weapons," said Pentagon spokesman Navy Captain Jeff Davis.
"This involved a specific aircraft, a specific hangar -- both of which we know to be associated with chemical weapon use."
The White House late yesterday said the preparations were similar to those undertaken by the regime of Syrian President Bashar al-Assad ahead of an apparent chemical attack on a rebel-held town in April.
The White House warned Assad that he would pay a "heavy price" if his regime went ahead with another chemical weapons attack, which "would likely result in the mass murder of civilians, including innocent children."
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Russia, which has supported the Syrian regime since 2015 with air strikes against what it says are Islamist extremists, voiced anger at the tough rhetoric.
"We consider such threats against the Syrian leadership to be unacceptable," Kremlin spokesman Dmitry Peskov told journalists.