Video posted on the Telegram messenger app by Baza, a channel linked to Russian security services, showed a large aircraft falling towards the ground and exploding in a vast fireball.
The Il-76 is a military transport aircraft designed to airlift troops, cargo, military equipment and weapons. It has a normal crew of five people, and can carry up to 90 passengers.
Local governor Vyacheslav Gladkov said that an unspecified "incident" had occurred in the region's Korochansky district, northeast of Belgorod city, and that he was going to inspect the site. He said investigators and emergency workers were already on the scene.
The Kremlin said in response to a reporter's question that it was looking into the situation.
Belgorod region, which borders Ukraine, has come under frequent attack from Ukraine in recent months, including a December missile strike which killed 25 people.
(Writing by Felix Light; Editing by Mark Trevelyan)
