U.S. Defense Secretary Jim Mattis has warned Syria that it would be "very unwise" for government forces to use weaponized gas, and slammed Russian support for Damascus.
Mattis made the remark on March 11 to reporters accompanying him on a trip to Oman, amid reports that President Bashar al-Assad's forces have recently used chlorine gas in a rebel-held region of eastern Ghouta, near Damascus.
The Syrian government denies the allegations.
The Pentagon chief stopped short of threatening to retaliate against Syrian forces if a chlorine attack were confirmed, but said U.S. President Donald Trump had "full political maneuver room" to take whatever decision he believed was appropriate.
In April 2017, Trump ordered a missile strike against a Syrian air base after Washington said the facility was used to launch a sarin nerve-gas attack on the rebel-held town of Khan Sheikhun, killing dozens of civilians.
The besieged eastern Ghouta region is the last opposition-controlled pocket near the Syrian capital.
For nearly three weeks, government forces have pounded it in an assault that has killed more than 1,000 civilians, according to activists.
Mattis said Russia, which has given Assad's government crucial support throughout the Syrian conflict, could be complicit in the Syrian government's strikes on civilians.