It is still not known what caused the explosion at the Raleigh County mine operated by Massey’s Performance Coal division. The initial six reported dead shortly after the 3.30pm incident rose to 12 around midnight local time and by Tuesday morning had spiked to 25.

As of 10.30am local time Tuesday, eleven workers had been identified. None of the miners’ names have yet been released, but the family of 62-year-old Benny Willingham told various news outlets that their relative, confirmed to be among the fallen, was just five weeks from retiring.

Federal officials have confirmed that the incident occurred during shift change Monday, with day shift crews exiting the mine.

"It does not appear that any of the individuals made it to a rescue chamber," US Mine Safety and Health Administration coal administrator Kevin Stricklin said.

"The situation is dire."

Stricklin had said in an earlier press conference that officials were hoping crews had made it to the refuge chambers Massey had placed underground, any of which would offer survival equipment and supplies. However, one of the two nearby units reached during initial rescue efforts was empty, and toxic gas build-up kept them from reaching any others.

Massey officials announced overnight that rescue efforts for the remaining four workers were suspended due to conditions underground, with methane concentrations also prohibiting the those crews from proceeding further.

“Rescue efforts will resume as soon as conditions allow,” the company said.

Work has begun on a series of surface drill holes, which will be sunk approximately 1,200 feet into the mine for ventilation and air sample collection. West Virginia governor Joe Manchin told media outlets early Tuesday morning that none of the holes are expected to reach their destinations until sometime in the evening.

Stricklin said it is not known how long the process will take.

"I think it's a dire situation, but I do think that it is a rescue operation and it will be that way until we confirm that these four additional people are not living," he said.

"Basically all we have left is hope."

US secretary of labor Hilda Solis said that the situation has been a “heartbreak”

“Twenty-five hardworking men died needlessly in a mine [Monday]. I pledge that their deaths will not be in vain,” she said.

“The federal Mine Safety and Health Administration will investigate this tragedy, and take action. Miners should never have to sacrifice their lives for their livelihood.”

The 200-employee mine, owned by Massey Energy since 1994, produced 1.2 million tons of coal in 2009.

The Upper Big Branch explosion is the worst fatal US mining incident since December 1984, when 27 workers died in a fire at the Wilberg operation in Utah.

Prior to the incident, US mining had recorded two coal deaths in 2010. A record low of 34 deaths occurred in all of 2009.