Assad's forces have instead "evolved" their chemical weapons and made continued occasional use of them in smaller amounts since a deadly attack last April that drew a United States missile strike on a Syrian air base, the officials told reporters in a briefing.
United States officials believe the Assad regime continues to use chemical weapons to terrorize civilians, to change the demographic make up of certain regions, and to compensate for a shortfall in troops.
Years of efforts by two USA presidents have failed to end the harrowing reports on chemical weapons use in Syria. After another alleged attack in April 2017, President Donald Trump ordered a retaliatory missile strike, but 10 months later, the US and global observers say the weapons are still used.
"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.
The official was referring to Russia's media offensive to deny its connection to chemical weapons use in Syria.
"Russia ultimately bears responsibility for the victims in Eastern Ghouta and countless other Syrians targeted with chemical weapons since Russian Federation became involved in Syria", Mr Tillerson said.
But while the Trump administration has resolved to punish Syria's use of chemical weapons with military force, a change in tactics from Damascus may complicate things.
"It will spread if we don't do something", warned an official who wanted to remain anonymous.
"Deterrence is something that you have to continually work at", said the second official.
Senior US officials claimed the Syrian government may be developing new types of chemical weapons.
Rather, the officials said Assad's forces are using the same chemicals - chlorine and sarin - but in more sophisticated ways, potentially to evade worldwide accountability by making the origins of attacks harder to trace.
And they said the Trump administration wouldn't rule out another military strike against President Bashar al-Assad's military if he doesn't stop the attacks. "And that use will spread to US shores".
Reports of chemical attacks have continued to stream in from Syria, including as recently as Thursday, when rescue workers in the rebel-held Damascus suburb of Douma reported what they described as a suspected chlorine gas attack that injured a number of civilians.