The White House said the Syrian government appears to be preparing to launch another chemical-weapons attack and it warned Syria that it "will pay a price" if that occurs.
The White House warning late on June 26 said preparations by Syria 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.
"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria," spokesman Sean Spicer said.
"If, however, [Syrian President Bashar al-Assad] conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."
Trump ordered the strike on the Shayrat airfield in April after what he said was a poison gas attack by Assad's government that killed at least 70 people -- a charge Syria denied.
The strike put Washington in confrontation with Russia, which has backed Assad with air strikes in his six-year civil war with rebels.
U.S. officials called the April intervention a "one-off" move intended to deter future chemical attacks and not an expansion of the U.S. role in Syria.