Syria has denied continued use of chemical weapons but Secretary of State Rex Tillerson said Assad's forces had launched a new round of attacks in East Ghouta where civilians were reportedly suffocating and dying as a result of military strikes.
Officials said that Assad may have secretly kept part of Syria's stockpile, and recent attacks by Damascus have led them to believe that Syria may be developing new weapons entirely, Reuters reported.
Raising the alarm about the continued threat, US officials said it was "highly likely" that Assad kept a hidden stockpile of chemical weapons after 2013 that he failed to properly disclose.
An agreement between the USA in Russian Federation in 2013 bound Moscow to remove all chemical weapons from Syria, but as repeated instances of chemical weapons attacks show, that was simply not the case.
Defense Secretary James Mattis on Friday warned the Syrian regime to think carefully before using chemical weapons again.
Two senior US officials warned on Friday that Syria's continued chemical warfare continue could serve as an opening to a chemical attack on the United States, security website The Cipher Brief reported Friday.More news: USA places Hamas chief Haniya on terror blacklist
"We reserve the right to use military force to prevent or deter the use of chemical weapons", one official said, while declining to specify how serious a chemical attack would have to be to draw a fresh USA military response.
Former US ambassador to Turkey and Washington Institute fellow James Jeffrey told BI that there have been persistent reports of chlorine attacks in Syria since 2013, and it's not as clearly banned by worldwide agreements as sarin.
The news came shortly after scientists for the Organisation for the Prohibition of Chemical Weapons said they had linked samples from the 2013 attack to the Assad government's stockpile, suggesting the government had been behind the attack all along.
The Syrian army and government have consistently denied using chlorine or other chemical weapons during the war and has said it cooperates with global investigations. "Using military force is something that is still considered".
The statement comes in the wake of suspected sarin and chlorine attacks - including one unconfirmed attack in the rebel-held town of Douma on Thursday. The Islamic State militant group continues to use them, they said, although the militants' arms are said to be more rudimentary.
Rescue workers and medical groups working in the rebel-held enclave of Eastern Ghouta, near Damascus, have accused government forces of using chlorine gas three times over the last month, including early on Thursday.
The U.N. says more than 270,000 people have been displaced in Idlib because of the government onslaught since December 15.