The statement is the closest officials have come to confirming mounting suspicions that the plane was downed by a Russian air-defense system.

It comes as the head of Russia's civil aviation agency says the situation in Grozny, the plane's destination, was "very complicated" at the time.

The Kremlin has said that the Russian president "apologized for the tragic incident" and expressed his "deep and sincere condolences" to the victims' families.