Reports of the latest chemical weapons attack comes days after the Trump administration accused Syrian President Bashar Al-Assad's government of producing and using "new kinds of weapons" to deliver deadly chemicals, despite committing to abolish its chemical weapons program in 2013.
The Syrian government rejected USA allegations on Saturday that it had used chemical weapons to attack rebel-held Eastern Ghouta last month.
Administration officials said Trump has not ruled out additional military action to deter chemical attacks or to punish Assad, though they did not suggest any action was imminent. He stressed that the USA did not have evidence of sarin gas use.
If the worldwide community does not act quickly to step up pressure on Assad, Syria's chemical weapons could spread beyond Syria and possibly even "to USA shores", a second official said.
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.
"You all have seen how we reacted to that, so the Syrian government would be ill-advised to go back to violating the chemical convention", Mattis said.
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.
Syria had agreed in 2013 to give up its chemical weapons to the Organization for the Prohibition of Chemical Weapons (OPCW).
The United States frequently accuses the Syrian government of developing and using chemical weapons in order to "justify its illegal and criminal acts" in the war-ravaged country, says an American analyst.
A Syrian couple grieving for their children murdered in a chemical weapons attack on Aleppo. Make a donation and help pay for our journalism.