The warplane was downed late on Saturday when it came under militant fire from the ground in the western Hama province where the Syrian government has been fighting Daesh and the al-Qaeda-linked Nusra Front, both outlawed in many countries, including Russia.
"The jet was shot down during an attack by terrorists in the province of Hama by a US Man-Portable Air-Defense System Stinger," the source said.
The source added that the Syrian army had suspended flights from the airfield in Hama following the incident.
One pilot of the MiG-21US jet managed to eject safely while the second died in a failed emergency landing, according to sources.