The newswould mean the number of Russian troops in Syria has doubled since Moscow began bombing the evil terror group in September, according to US officials.
Meanwhile Russia has flooded Syria with missile systems to protect its forces from hijacked fighter jets.
Colonel General Viktor Bondarev, head of the country's air force, said planes could be taken over in countries neighbouring Syria and used to attack Russian forces.
He said: "We have calculated all possible threats. We have sent not only fighter jets, bombers and helicopters, but also missile systems," Bondarev told Komsomolskaya Pravda newspaper.
"We must be ready."
His comments come as Number 10 grounded UK flights out of Sharm el-Sheikh amid fears the Russian jet downed in the Sinai Desert on Saturday was blown up by an ISIS bomb.
The US has also claimed the number of bases used by Putin's killing machines in the war-torn nation has grown.
Hundreds of ISIS sickos are thought to have already been wiped out – along with the group's arms supplies.
A spokesman for the Russian foreign ministry refused to comment on the number of troops in Syria yesterday.
The US officials also claimed several Russians have been killed – but failed to give exact numbers.
The only death reported by the Russian government is a serviceman who committed suicide.