The Khe Tam coal mine in Vietnam’s coal-rich province of Quang Ninh is owned and operated by the military-run Northeast Coal Corporation which is in turn controlled by a government umbrella organization for the coal industry known as Vinacom.

Six workers were reportedly installing an electrical system 150m down the pit when the blast ripped through the mine, killing four instantly and trapping their bodies inside the collapsed area.

Vietnamese news service Tuoi Tre said it took five-and-a-half hours to recover the first body and the last three were not retrieved until Monday evening.

The report said the victims had been returned to their families for burial with $US288 in support compensation from provincial authorities.

An investigation into the cause of the explosion is ongoing but Vietnam’s Thanh Nien newspaper has speculated the explosion may have been caused by human error in the work process.

Dozens of workers have been killed in the past decade in Quang Ninh mines which account for some 90% of the country’s coal production.

Vietnam produced about 50 million tonnes of coal in 2010, mostly anthracite, from about 30 open pit and 30 underground mines.