The United States has received numerous reports that appear consistent with chemical exposure after an attack by Syrian government forces in northwest Syria, but it has made no definitive conclusion as to whether they used chemical weapons, the State Department has said.
"We do have numerous sources including interviews with those present during the attack that did report that a number of opposition fighters were taken to local hospitals and presented symptoms that were consistent with chemical exposure," State Department spokeswoman Morgan Ortagus told reporters.
The State Department said on Tuesday it saw signs the Syrian government may be using chemical weapons, including what it called an alleged chlorine attack on Sunday in northwest Syria. The department warned that Washington and its allies would respond "quickly and appropriately" if this were proven.
"We know of course that this is a pattern of behaviour, unfortunately, by the Assad regime, but we don't have any definitive conclusions yet as we continue to investigate," Ms Ortagus said.
Syria's government denies using chemical weapons.
Rebels fighting on the mountainous western edge of Syria's last big rebel enclave of Idlib said on Sunday that the army had shelled them with poison gas, leading some to suffer choking symptoms. They said they had not documented the attack because they were under bombardment when it occurred.
Morgan Ortagus said the location of the alleged attack was "at the front lines, so it does make access to that site limited."
The Trump administration has twice bombed Syria over Assad's alleged use of chemical weapons, in April 2017 and April 2018.
Syrian President Bashar al-Assad launched an offensive at the end of April on Idlib and parts of adjacent provinces, saying insurgents had broken a truce. This week, rebels rolled back some government advances on the main battlefront.
The US says it has been working to encourage a return to a ceasefire that had been in place since September.