The Pentagon says the United States has seen evidence of what appeared to be active preparations by Syrian government forces for a possible chemical-weapons attack.
Pentagon spokesman Captain Jeff Davis said on June 27 that activity was detected at Syria's Shayrat airfield -- the same Syrian airfield that was struck by a U.S. missile attack in April.
Davis said U.S. intelligence detected preparations involving "specific aircraft in a specific hangar, both of which we know to be associated with chemical-weapons use."
The White House said late on June 26 that preparations by Syria were similar to those undertaken before a suspected chemical attack on April 4 that prompted President Donald Trump to order a cruise-missile strike on the Syrian airfield.
"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."
The U.S. ambassador to the United Nations, Nikki Haley, said that Russia and Iran, Assad's key allies in Syria's civil war, would also be responsible if such an attack took place.
Haley told a U.S. House of Representatives hearing on June 27 that the "goal is at this point not just to send Assad a message but to send Russia and Iran a message that if this happens again we are putting you on notice."
Russia denounced the White House warning, with Kremlin spokesman Dmitry Peskov saying, "We consider such threats against the Syrian leadership to be unacceptable."
"I am not aware of any information about a threat that chemical weapons can be used," Peskov added.
Trump ordered the strike on Syria's 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 Damascus denied.
The strike put Washington in confrontation with Russia, which has backed Assad with air strikes in his 6-year-old 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.