The Soviet-made Mi-17 helicopter burst into flames after coming down from a height of 50 metres at around 8:30 pm (1730 GMT) not far from the Krumovo military airbase and Plovdiv's civilian airport.

"Two of the crew -- the commander and the co-pilot were killed in the fall of the helicopter. The third crew member -- the board mechanic -- was immediately transported to hospital, (he was) conscious," the ministry said in a statement.

The cause of the accident was not immediately known, it added.

The helicopter was Bulgaria's only operational Mi-17 chopper and mainly used for firefighting in difficult terrains.

The accident is the second deadly crash involving a Bulgarian military helicopter in the last 12 months.

Last June a Panther-565 of the Bulgarian navy plunged into the Black Sea during a military exercise off the coast of Varna, killing the pilot.

A risky manoeuver and bad weather were then cited as causes of the accident but Bulgarian President Rumen Radev, a former airforce chief, also blamed insufficient flight hours.

Bulgaria's outdated military equipment, most of which was bought during communism and is Soviet-made, has often caused tensions in the military, with pilots at the country's major Graf Ignatievo airbase refusing to fly MiG-29 jets last October over safety concerns.

Public tenders for new equipment have been sidetracked for years. But last Friday Bulgaria's parliament approved plans to spend more than 3.2 billion leva (1.6 billion euros, $1.9 billion) on new fighter jets and armoured carriers.