The United States saw what appeared to be active Syrian preparations for a possible chemical weapons attack at Shayrat airfield, the same Syrian airfield the United States struck in April, Pentagon spokesman Captain Jeff Davis said on Tuesday.
"This involved specific aircraft in a specific hangar, both of which we know to be associated with chemical weapons use," Davis said, speaking by phone from Washington.
Syrian regime leader Bashar al Assad may be preparing a chemical weapons attack that would result in the "mass murder" of civilians, the White House had said earlier, warning the regime would pay a "heavy price" if it goes ahead.
"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria [Daesh]," spokesman Sean Spicer said in a statement.
"If, however, Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."
The White House statement said preparations by the Syrian regime were similar to those undertaken before an April 4 chemical attack that killed dozens of civilians and prompted President Donald Trump to order a cruise missile strike on a Syrian air base.
Trump ordered the strike on the Shayrat airfield in Syria in April in reaction to what Washington said was a poison gas attack by Assad's regime that killed at least 70 people in rebel-held territory. The Syrian regime denied it carried out the attack.
The strike put Washington in confrontation with Russia, which has advisers in Syria aiding its close ally, Assad.
US officials at the time called the intervention a "one-off" intended to deter future chemical weapons attacks and not an expansion of the US role in the Syrian war.