Last April, the USA launched several dozen Tomahawk cruise missiles at a Syrian air base in response to what it called illegal Syrian use of chemical weapons.
He noted however that the United States did not yet have proof the chemical weapon had been used.
The two officials, speaking anonymously as a condition of the briefing, further warned that if Syria's behavior is allowed to continue, it could serve as a green light to other bad actors that could spell a chemical attack on USA shores.
USA officials said the characteristics of the recent alleged attacks suggest Syria was producing chemical weapons despite a 2013 deal to destroy its program, and that it was "highly likely" that Syria kept a stockpile of weapons.
The Assad regime has been repeatedly accused of using chemical weapons on civilians.
"We're on the record and you all have seen how we reacted to that, so they would be ill-advised to go back to violating the chemical convention", Mattis told reporters at an off-camera gaggle at the Pentagon, according to multiple reports.
But with Russian and Iranian influence growing in Syria and posing a direct threat to U.S. foreign policy interests, it's possible that the Trump administration may look to make a statement that it's not buying Russia's excuses anymore.
USA officials on Thursday said the Syrian military was shifting tactics to try to hide its role in using such weapons.
Recent attacks in Syria suggest that new weapons and methods of delivery are being developed, possibly to prevent the origin from being traced.
A deadly sarin attack on a rebel-held area in April prompted Mr Trump to order a missile strike previous year on the Shayrat airbase, from which the Syrian operation is said to have been launched.
Nauert's comments follow a suspected chlorine gas attack in the rebel-held territory of Eastern Ghouta, on the outskirts of Damascus, on Thursday.
If left unchecked, the United States officials added, Assad could use small chemical attacks in the hopes to redress the power imbalance in rebel-held areas as an "instrument of terror". The Islamic State militant group continues to use them, they said, although the militants' arms are said to be more rudimentary.
Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Russian Federation and the United States. Nearly all independent analysts and investigators agree the Syrian regime is the culprit of the chemical attacks.
Mr Burt referred to three specific reports of chlorine attacks by Syrian regime forces in Eastern Ghouta so far in 2018.
Syria's war has killed more than 340,000 people and displaced millions since it began in March 2011 with the brutal repression of anti-government protests.