Monday's attack hit the Tiyas airbase, known as T4, near the city of Homs.
Syria initially said the West was involved. The US and France threatened to respond to the alleged chemical attack, but deny striking the base.
Israel has previously hit Syrian targets. It has not commented.
US President Donald Trump said there would be a "big price to pay" after the chemical attack in Douma, in the Eastern Ghouta region, near the capital, Damascus.
He branded Syria's President Bashar al-Assad an "animal".
Meanwhile the last rebel fighters in Douma have surrendered and are being bussed out of the enclave after reaching a deal with the government.
Mr Trump and France's President Emmanuel Macron issued a joint statement on Sunday vowing to "co-ordinate a strong, joint response" to the alleged attack.
Suspected Syria chemical attack kills scores
But US officials said it did not launch a missile attack.
"At this time, the Department of Defense is not conducting air strikes in Syria," the Pentagon said in a statement.
"However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable."
France also issued a denial.
Syria's state news agency Sana initially called the reported missile strike on the Tiyas airfield a "suspected US attack," but later dropped the reference to the US.
In April 2017, the US fired 59 Tomahawk cruise missiles at Syria's Shayrat military airfield in response to a chemical weapons attack on a different rebel-held town, Khan Sheikhoun.
BBC
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