The jet crashed in the north of Vietnam last Thursday, killing two pilots.
"The reason of the crash is probably the fact that the aircraft was upgraded in Ukraine in 2007 and was being serviced by non-authorized Ukrainian companies," the source said.
Su-17/22 is a variable-sweep wing fighter. These aircraft, originally developed by the JSC Sukhoi Company (then—Sukhoi Design Bureau) in the 1960s, are currently operated by military in Angola, Iran, Poland and Vietnam.