Eyewitnesses said they heard the engine of the MI-17 stall and then saw the aircraft hit the ground and explode.

The helicopter had came in from a base at Kroumovo, in the south, to help more than 200 firefighters who had battled a major blaze throughout the night on the flanks of Mount Pirin, part of the chain of the same name.

The defense ministry said the dead were three Bulgarian airforce members and a firefighter.

"All four were specialists with experience," said Bulgarian Defense Minister Nikolai Svinarov who cancelled a planned visit to Romania on Thursday and went to the scene of the accident.

The government has appointed a commission to investigate the accident and Interior Minister Georgi Petkanov told reporters: "The black box of the aircraft has been found and will reveal the reason."

Unofficial explanations so far include soaring temperatures, the wind and poor visibility due to the smoke rising from the fire.

The agriculture ministry said it believed arsonists started the fire, which has been brought under control.

MI-17 are used to fight fires in inaccessible areas and can carry up to two cubic metres (70 cubic feet) of water.

TERRA.WIRE