The Syrian government has moved most of its combat planes to Russian military bases in anticipation of further US strikes, American officials have said.
The relocation of its warplanes to Hmeimim air base and the adjacent Bassel al-Assad International Airport began shortly after the US's April 6 Tomahawk cruise missile strike on Shayrat airfield, which were carried out in retaliation for a chemical weapons attack that was believed to have been launched from that airfield.
The strikes were reported to have destroyed 24 aircraft, but planes continued to fly bombing raids out of the airfield just hours later.
The heavily fortified Hmeimim base in the coastal Syrian city of Tartus is under the protection of the Russian S-400 air-defence system, which has a range of 150 miles.
Since the defence system was installed in 2015, the actions of Western and Turkish warplanes in support of rebel forces has been limited.
The US has not ruled out further strikes, however doing so in future would now risk hitting one of the 30 Russian aircraft or 15 helicopter gun as well, as hundreds of personnel, stationed at Hmeimim.
Washington has feared any confrontation with Moscow over Syria could escalate into a full-blown conflict between the former Cold War foes.
The reports came as the Organisation of the Prohibition of Chemical Weapons (OPCW), the UN’s chemical watchdog, said "incontrovertible" test results show sarin gas or a similar substance was used in the April 4 attack.
Samples from 10 victims who had been treated in Turkey were analysed at four OPCW laboratories.
British scientists had earlier presented similar findings to the UN security council.
The Russian military is questioning the conclusions, however. Maj Gen Igor Konashenkov, defence ministry spokesman, issued a statement on Thursday questioning how the samples were collected and how the analysis could have been done so quickly.
Moscow and Tehran jointly put forward a proposal to the OPCW to launch a new investigation.
Maj Gen Konashenkov said inspectors should be sent to the site, a rebel-held area south of Idlib, as well as Shayrat airbase, to "verify allegations concerning the storage of chemical weapons" there.
But the move, which was overwhelmingly rejected yesterday, had "attempted to undercut" the OPCW's existing fact-finding mission, the British delegation said.
"Needless to say - #OPCW investigation continues" and "the UK fully supports it," they wrote in a tweet.