At least 72 workers were in the mine in Antioquia province when the explosion occurred on Wednesday.

Initial reports said an accumulation of methane gas had caused the blast.

Search suspended

A team of rescue workers entered the mine on Thursday afternoon but a build-up of toxic gases hampered their work.

After getting 180 metres into the mine, the search was suspended to allow gas levels to dissipate.

"Until now, the rescue operation has been sporadic," Al Jazeera's Teresa Bo, reporting from the San Fernando mine, said.

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"High temperatures inside the mine increases the concentration of gas, making this rescue operation a very dangerous one. Rescue workers have to come in and out constantly."

She said the mining industry in Colombia has expanded in recent years.

"But people say it has not improved the living conditions and that's why workers have to go inside the mines that many don't consider safe enough - it's the only way they have to survive," our correspondent said.

The state Institute of Geology and Mining said experts inspected the mine last month and found it "didn't have gas detection devices, one of the fundamental requirements to guarantee safety in case of an explosion".

Common phenomenon

Build-up of methane gas is common in coal mines and if ventilation systems are not working properly, sparks can cause the gas to explode.

Wednesday's blast collapsed parts of an access tunnel that is more than 1.6km-long and drops to a depth of 150 metres.

Two injured miners managed to escape from the mine, John Rendon, the provincial disaster co-ordinator, said.

A 22-man army special forces rescue team was being dispatched to the mine to help and at least 100 rescue workers were at the scene.

Mining accidents are common in Colombia, the world's fifth largest coal exporter.

Last year, a methane-gas explosion killed eight workers in another mine in Antioquia.

In one of the worst disasters of its kind in a decade, 31 miners were killed in a blast in Norte de Santander in 2007.