"We're certainly seeing the evolution of allegations into new kinds of weapons that suggests an ongoing production capability" related to sarin and chlorine weapons, one official said. "They clearly think they can get away with this if they keep it under a certain level."
The comments indicate mounting American concern that Syrian authorities may be trying to rebuild their chemical weapons program despite years of international censure — and that support from Russia may be helping them do so.
The senior U.S. officials, speaking to journalists on the condition of anonymity because they were not authorized to speak publicly, said there did not appear to have been any "large use" of banned munitions since April.
But there have been repeated reports of smaller chemical attacks on civilian areas. On one day last month, two separate chlorine attacks were reported in rebel-held towns.
State Department spokesman Edgar Vasquez said the United States was "extremely concerned" about another report of chlorine gas use Thursday in Eastern Ghouta, an opposition-controlled suburb of Damascus that has been the site of previous chemical attacks.
"We will continue to seek accountability through all available diplomatic mechanisms, including the United Nations Security Council and the Organization for the Prohibition of Chemical Weapons, for the confirmed use of chemical weapons by any party," Vasquez said in a statement.
The senior U.S. officials declined to provide details about the U.S. assessment of recent chemical attacks but suggested there had been credible reports of at least one sarin attack.
Officials said the purported use of chemical weapons was an attempt to compensate for dwindling military resources as the Assad government seeks to retake the large swaths of territory it has lost in its prolonged civil war and grows more reliant on backers in Moscow and Tehran.
"They don't have the manpower to be strong everywhere at once," a second official said.
The weapons are also a means, the officials said, to strike fear among residents of opposition-controlled areas, possibly with the goal of reshaping the demographic makeup of those zones and deterring the return of refugees who may oppose the government.
The officials said Syrian authorities appeared to have embraced means for launching chemical strikes from the ground rather than the air attacks they have used in the past, presumably because the ground-based operations are harder to detect.
Facing the likelihood of renewed chemical attacks, senior U.S. officials called on international allies to intensify pressure on the Assad government.
They singled out Russia, echoing comments from Secretary of State Rex Tillerson and other U.S. leaders who have said Moscow bears responsibility for the chemical attacks.
But the officials stopped short of promising additional military action to halt Syria's use of chemical weapons.
"The president showed last April he's willing to look at all the options . . . and using military force is something he'll still consider doing," the second official said. But he declined to say whether a larger attack might prompt another military action.
The officials also said the Islamic State, also known as ISIS, has continued to use chemical weapons in a much cruder, limited way.
"If the international community does not take action now . . . we will see more chemical weapons use, not just by Syria but by non-state actors such as ISIS and beyond," the first official said. "And that use will spread to U.S. shores."