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.


