Syria’s government appears to have taken a US warning not to launch a chemical weapons attack seriously, Defence Secretary James Mattis says.
The White House said similar activities had been seen at an airbase before the nerve agent Sarin was allegedly dropped on rebel-held Khan Sheikhoun in April.
President Bashar al-Assad’s military would “pay a very heavy price” if an attack took place, it warned.
Damascus dismissed the allegation, but Gen Mattis noted: “They didn’t do it.”
Mr Assad has said the incident in Khan Sheikhoun, which activists say killed more than 80 people and prompted a retaliatory US missile strike, was a “fabrication”.
He has insisted that his forces destroyed their entire chemical arsenal under a deal brokered by the US and Russia after a Sarin attack outside Damascus in 2013.
Pentagon spokesman Capt Jeff Davis said on Tuesday that the US had recently observed activity at Shayrat airbase, about 25km (15 miles) south-east of the city of Homs, which “indicated preparations for possible use of chemical weapons”.
The activity involved “specific aircraft in a special hangar, both of which we know to be associated with chemical weapons use”, he added.
President Donald Trump ordered a missile strike on Shayrat in April after US officials concluded that the facility was where the Syrian Air Force jet which allegedly attacked Khan Sheikhoun had been armed with a Sarin-filled bomb.
The White House warned Mr Assad on Monday that if he “conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price”.
“It appears that they took the warning seriously,” Gen Mattis told journalists on a flight to Brussels on Wednesday. “They didn’t do it.”
But he cautioned that “Assad’s chemical programme goes far beyond one airfield”.
Last month, a Western intelligence agency told the BBC that chemical and biological munitions were still being produced at three main sites near Damascus and Hama run by the Scientific Studies and Research Centre (SSRC).
Russia, Mr Assad’s main ally, denounced the White House statement.
“I am not aware of any information about a threat that chemical weapons can be used,” Kremlin spokesman Dmitry Peskov said. “Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable.”
Mr Peskov also criticised the Trump administration for using the phrase “another chemical weapons attack”, arguing that an independent investigation into what happened at Khan Sheikhoun had yet to be carried out.
Video posted following the alleged air strike showed people struggling to breathe and foaming at the mouth – symptoms consistent with nerve agent poisoning.
The Organisation for the Prevention of Chemical Weapons (OPCW) has said tests on samples collected from victims proved “incontrovertibly” that they were exposed to Sarin or a Sarin-like substance.
While President Assad said the incident was faked, Russia said a Syrian jet bombed a rebel chemical munitions depot and released toxic agents into the air.