Rescuers are fighting rising odds to save 65 Mexican miners trapped in underground chambers since a gas blast on Sunday.
As relatives held bonfire vigils overnight, there were rumours that rescuers had heard knocking in the Pasta de Conchos coal mine in the northern state of Coahuila, owned by Grupo Mexico.
But two days after the miners were trapped underground, there were still no signs of life and rescuers were a long way from the chambers where they were buried.
A rescue team got within 55 yards of where two men had been working on a conveyor belt when Sunday's gas explosion occurred, but poor ventilation and the risk of rock falls hampered efforts to save them.
Rescuers were using only hand tools for fear power equipment might spark another blast.
The rest of the men were thought to be in groups up to two kilometres into the mine, near the town of San Juan de Sabinas, 100 kilometres southwest of Eagle Pass, Texas.
Mine official Ruben Escudero told reporters that digging teams were working rotating six-hour shifts and that a group of specialist rescuers were on their way from the United States.
But he said they had yet to widen an air ventilation shaft enough to send rescuers and their equipment into the collapsed chambers where the bulk of the miners were trapped, and that there was a high risk more of the mine would give way.
The miners' oxygen tanks have almost certainly run out, but mine operators said air pockets might still keep them alive.