The US says it has identified "potential preparations" for another chemical attack in Syria, and issued a stark warning to the Bashar al-Assad government, a media report said.
The White House on Monday said the Assad regime's activities were similar to those made before a suspected chemical attack in April, the BBC reported.
Dozens died in that attack and prompted President Donald Trump to order a strike against a Syrian air base.
The US statement warned Syrian President Assad of "a heavy price" if another strike occurred. It said "another chemical weapons attack by the Assad regime" was likely to result "in the mass murder of civilians", the BBC report said.
White House spokesperson Sean Spicer said: "As we have previously stated, the US is in Syria to eliminate the Islamic State (IS). If, however, Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."
Dozens of civilians, including many children, died in a suspected nerve gas attack in the town of Khan Sheikhoun in Idlib, on April 4.
In response, US Navy ships in the Mediterranean fired 59 Tomahawk cruise missiles at Shayrat airfield, in western Homs, which it said was used to store chemical weapons.