US intelligence experts assess Syrian President Bashar al-Assad's military is probably hoarding the weapons in munitions depots at Shayrat airfield near Homs in central Syria, the US military's Central Command spokesman Colonel John Thomas said.
President Donald Trump ordered a massive military strike on the air base last week, in retaliation for a "barbaric" chemical attack he blamed on Assad.
"We suspected there was a significant probability there could be other chemical weapons which would be ready to go, weaponized in those facilities, and so we didn't strike those," Thomas told reporters.
Though the United States is confident Assad is stockpiling chemical weapons, intelligence analysts are not certain what these are.
Under a 2013 Moscow-brokered deal, Assad was supposed to have dismantled Syria's chemical weapons arsenal and ship it to Russia.
The Pentagon has said it is sure Assad unleashed a chemical attack on April 4 that killed at least 87 civilians in the rebel-held Syrian town of Khan Sheikhun.
"Our advice was to make sure we didn't inflict any greater damage by touching any of the chemical weapons in the area," Thomas added.
The Pentagon says the strike destroyed more than 20 Syrian jets but the military has come under criticism for not targeting Shayrat's runways, amid reports regime planes were again using the base hours after the attack by 59 Tomahawk cruise missiles.
Thomas said the runways were deliberately avoided because the United States was trying to draw a clear line that its military action was in response to the suspected chemical attack, and not signal a willingness to get more involved in Syria's brutal civil war.
Moscow reacted furiously to the US strike and immediately threatened to suspend a vital hotline established to avoid mishaps between the two powers in Syria.
Thomas refused to confirm whether the so-called "deconfliction line" remains operational, but said that even without it, "there are still ways for the pilots to hail each other."
While US and Russian pilots can use radar to see each other, the lack of a direct line of communication would further heighten the risks of misunderstandings or accidents in Syria's already crowded skies.
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