A spokesman for mine owners Grupo Mexico said that while no bodies were found, air tests in a section of the Pasta de Conchos mine where men were thought to have been working showed there was not enough oxygen to sustain life.

Sunday's blast dislodged huge piles of rocks inside the mine and slowed miners trying to dig through to their missing colleagues.

Stricken relatives demanded Francisco Salazar, the labour minister, give them the names of the dead and called him a liar when he said he could not.

The minister said five more holes would be drilled in places where miners were thought to be trapped so that the air could be tested there.

Not enough oxygen

Xavier Garcia, head of Mexican mining operations at Grupo Mexico, said rubble weighing about 70 tonnes sat in a tunnel where two miners were believed to have been working when the methane gas erupted.

Men digging with spades and even their bare hands found no sign of the pair.

Garcia said: "We were surprised. The explosion was much bigger than we had imagined."

Rescuers were still about 700m from where most of the miners were believed to be. The rescuers were in an area that still had breathable air but the level of methane gas was increasing as they dug.

Since the accident, there has been no contact with the missing miners, who had only six hours' worth of oxygen with them.