The US says it has identified “potential preparations” for another chemical attack in Syria, and issued a stark warning to the Syrian government.
The White House said the activities were similar to those made before a suspected chemical attack in April.
Dozens died in that attack and prompted President Donald Trump to order a strike against a Syrian air base.
The US statement warned President Bashar al-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 statement added: “As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria. If, however, Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price.”
President Assad has denied his forces were behind the suspected nerve gas attack in the rebel-held north-western town of Khan Sheikhoun in April.
Dozens of civilians, including many children, died.
In response, US Navy ships in the Mediterranean fired 59 Tomahawk cruise missiles at Shayrat airfield, in western Homs province, which it said was used to store chemical weapons.
The latest development comes amid increasing tension in the region between the US and President Assad’s ally, Russia.
The Kremlin recently warned the US-led coalition that it would view its aircraft as targets after the US shot down a Syrian military plane.
Syria’s six-year civil war has left more than 300,000 people dead and created more than five million refugees.
Militant Islamist groups including so-called Islamic State (IS) have used the conflict to seize swathes of land.
Warplanes from a US-led coalition are supporting an alliance of Arab and Kurdish militias as they carry out an assault on the group’s stronghold of Raqqa.