The movement of the aircraft to the air base at Bassel Al-Assad International Airport began shortly after the US's April 6 Tomahawk cruise missile strike
on Sharat air base, which destroyed some 24 Syrian warplanes in retaliation for a chemical weapons attack
that the US says Syria launched from that airfield.
The move places the Syrian aircraft in close proximity to Russia's Khmeimim Air Base -- where the majority of Russian air forces helping ally Syrian President Bashar al-Assad's regime are based -- in Latakia Governorate, Syria.
One US defense official said that the warplanes moved appeared to be most if not all of the Syrian government's operational aircraft.
The Khmeimim base, along with a naval facility in Tartus, is one of the two of the primary Russian military installations in Syria. The Russians have reportedly stationed advanced anti-aircraft missile systems at the base.
The regime in Damascus may be calculating that the US would be more reluctant to strike in close proximity to the Russian troops and their anti-aircraft systems.
The US warned
Moscow via a pre-established military-to-military communications channel in advance of its April 6 cruise missile strike in order to prevent any Russian casualties. It came days after Syria's rebel-held Khan Sheikhoun area of Idlib province experience a chemical weapons attack that killed more than 70 civilians.
The director general for the Organization for the Prohibition of Chemical Weapons, Ambassador Ahmet Üzümcü, said Wednesday that analysis of samples from victims of the Khan Sheikhoun attack showed "incontrovertible" results that "sarin or a sarin-like substance" was used in the attack.
US defense officials have previously said that the its retaliatory strike incapacitated some 20% of the regime's operational fixed-wing aircraft, making the preservation of the remaining planes of the utmost importance to Damascus.
"The Syrian Air Force is not in good shape. It's been worn down by years of combat plus some ... significant maintenance problems," Secretary of Defense Gen. James Mattis told reporters at the Pentagon.
The US has not ruled out additional strikes against the regime should it opt to use chemical weapons in the future.
"The Syrian regime should think long and hard before it again acts so recklessly in violation of international law against the use of chemical weapons," Mattis said, later adding: "If they use chemical weapons, they are going to pay a very, very stiff price."