President Donald Trump has not ruled out additional military action to deter attacks or punish Assad, administration officials said, although they did not suggest any action was imminent.
They emphasized that the United States was seeking a new way to hold chemical weapons-users accountable and wanted cooperation from Russia, Assad's patron, in pressuring him to end the attacks.
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. They said information gathered from recent alleged attacks also suggested that Assad retained a "continued production capacity" -- also banned under the 2013 deal.
Barrel bombs used earlier in the war to disperse chemicals indiscriminately, for example, have been replaced by ground-launched munitions, officials said.
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More recent attacks have involved both chlorine, which has nonchemical uses and is easier to acquire, and the more sophisticated chemical sarin, the officials said.
Though evidence-collection is different in the middle of a war zone, the officials said the US has a firm understanding of the extent of chemical use in Syria through a combination of intelligence, sample testing by third countries, and social media and other open-source information, the officials said.
Assad's government has denied using chemical weapons. Syria's chief ally, Russia, has claimed that the reports are false attempts to pressure Syria's government or provocations perpetrated by opposition groups.
Syria and Russia have dismissed the conclusions of the Joint Investigative Mission, an expert body set up by the United Nations and Organization for the Prohibition of Chemical Weapons, that Assad's government used chlorine gas in 2014 and 2015, and sarin in April 2017.
Use of such widely deplored weapons comes with great risk for Assad, raising questions about why he would take the chance.
But the officials said the US believes Assad's government sees chemical attacks as an effective way to terrorize rebels and sympathetic populations into fleeing, therefore altering the demographic balance in the Alawite heartland where Assad is trying to consolidate control.
Yet Syria's government isn't the only chemical weapons threat in the region, according to the officials. The Islamic State group continues to use them, they said, although the militants' arms are said to be more rudimentary.
Though IS no longer controls large parts of Syria or Iraq, the officials said the extremist group continues to use sulfur mustard, via artillery shells, and chlorine, delivered by improvised explosive devices.
The officials noted that the underlying chemicals are easy to acquire or produce, and said the US does not believe IS has gotten ahold of military stockpiles in either Iraq or Syria.