About 70-80 workers were 600-700m underground at the San Fernando mine at the time of the explosion, with two injured miners managing to escape according to Associated Press.

Antioquia province disaster coordinator John Rendon reportedly said the blast was caused by a methane gas buildup, with the explosion collapsing part of an access tunnel 1.6 kilometres long and up to 150m deep.

Few survivors are expected because of the dense level of gas underground with more than 60 miners still trapped.

Colombia Mines Minister Hernan Martinez told reporters he was concerned about the magnitude of the explosion and that there did not seem to be any gas detectors in the mine.

“The last inspection was June 9 and nothing irregular was found. However, we have to investigate because it is likely there was no gas detector," he told a news conference, according to AFP.

CNN reported the explosion occurred during a shift change at about 10:30pm local time, so more workers were underground than normal.

The small thermal coal mine produces around 240,000 tonnes a year, while the coal company that owns the mine has not yet been named in the various reports so far.

Five miners reportedly died in a flood at the mine two years ago.