The Kremlin on Thursday warned against speculating what may have caused an Azerbaijan Airlines flight to crash, killing 38 of the 67 on board, after an aviation expert said evidence indicated that a Russian anti-missile battery may have brought down the passenger plane.

Russian air defences downed an Azerbaijan Airlines plane that crashed in Kazakhstan, killing 38 people, four sources with knowledge of the preliminary findings of Azerbaijan's investigation into the disaster told Reuters on Thursday.

An Azerbaijan Airlines flight that crashed in Kazakhstan on Christmas Day was downed by a Russian air defence system, a preliminary investigation has found, according to reports.