In the wake of yet more suspected sarin and chlorine attacks blamed on the regime, Washington said it wants to send a message to Bashar al-Assad and Moscow that enough is enough.
The latest unconfirmed attack came on Thursday, in the rebel-held town of Douma. The Syrian Observatory for Human Rights said three people suffered respiratory problems after a rocket attack.
A senior US official told AFP that military options against Damascus similar to those launched in April 2017 were always on the table and "always feasible."
Trump "hasn't excluded anything" in the bid to halt the program, the official said. "Using military force is something that is still considered."
A second senior US official reported evidence that Assad's regime has a "ongoing production capability" focused on sarin and chlorine and is developing new ways to deploy the chemicals banned for weapons use.
"It looks like they are trying to evolve for either military reasons or to escape accountability. It is incredibly important to stop that before it gets off the ground," the official added.
Chlorine, not sarin
Aside from the threat to Syrian civilians, Washington is worried that a wave of well-documented chemical attacks -- systematically denied by Damascus and Moscow -- is undermining long-standing taboos on their use.
"We are convinced that if the international community does not take action now," the second official said, "we will see more chemical weapons use, not just by Syria but by non-state actors."
"That use will spread to US shores, if we cannot stop it."
The Assad regime appears to have altered course only slightly since the United States fired 59 Tomahawk cruise missiles at Syrian airfield in 2017 after a large chemical attack on rebel-held Khan Sheikhun.
Instead of dropping barrel bombs filled with chemical agents from helicopters, the officials said that mortars and other ground-based delivery systems were now being used.
"What they are trying to do as they tip toe along is testing, and what we are trying to say is that we continue to care about this," said the second official.
The chemical of choice has most often been industrial chlorine, which is easy to produce and legal to possess, rather than sarin, which is banned by the Chemical Weapons Convention.
"Deterrence is something that you have to continually work at," said the second official. "Deterrence isn't something I think you can expect to do one time then walk away and expect matters to take care of themselves."
On January 22, at least 21 people in Douma, including children, suffered breathing difficulties after a rocket attack on a besieged rebel enclave near Damascus.
An AFP correspondent at a hospital in the city saw people carrying babies wrapped in blankets, breathing through oxygen masks, some of them screaming.
Young girls and men sat on hospital beds, tears in their eyes, unable to stop coughing.
A doctor at the hospital who gave his first name as Bassil said patients were suffering "respiratory irritation, breathing difficulties, coughing and reddening of the eyes".
"We noticed that they smelled like bleach, or chlorine, and we stripped them of their clothes," he said.
Weapon of war
US 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.
There have been more than 260 reports of chemical attacks, some of which have been verified by UN-backed inspectors and attributed to the Assad regime.
President Barack Obama's administration struck a deal with Russia to remove some but not all of Syria's chemical weapons stockpile.
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