France said on Friday it was "deeply concerned" that Syria's government was flouting its pledges to stop using chemical weapons and Paris was working with its partners to shed light on recent suspected toxic gas attacks.
Defense Secretary Jim Mattis issued a stern warning on Friday to Syria, urging the regime to abstain from using chemical weapons.
"We have other reports from the battlefield from people who claim it's been used", Mattis said, adding the United States now had no proof to back the claims.
Washington said this week that it could consider military action against the Syrian government if it continued to use chemical weapons.
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The US officials further said that Syrian authorities appeared to have embraced ground-based means for launching chemical strikes rather than the air attacks because the ground-based operations are hard to detect.
The Trump administration is prepared to again take military action against Syrian government forces if necessary to deter the use of chemical weapons and is concerned they may be developing new methods to deliver such weapons, senior US officials said on Thursday.
Raising the alarm about the continued threat, USA officials said it was "highly likely" that Assad kept a hidden stockpile of chemical weapons after 2013 that he failed to properly disclose.
In April previous year, the USA struck a military base in central Syria with more than 50 Tomahawk missiles, in retaliation for an alleged chemical attack by the government forces on a rebel-held town in the northwestern province of Idlib. That strike followed a deadly chemical weapons attack by Assad's government. He said that non-governmental organizations and fighters on the ground in Syria had reported sarin use, and that the US was looking into the matter.