According to Agence France-Presse, the gas at the Santa Lucia mine near Pola de Gordon took the breath of the miners so quickly that there was no time for them to don protective breathing equipment.

Another five miners working in the 200m-long gallery about 700m underground were in stable condition at a regional hospital Tuesday. About 116 total miners were on shift at the mine complex.

None of those killed or injured have been identified, but survivor Jesus Gonzales told AFP that he was in the section workers were running to following a roof collapse. He speculated that the collapse may have pushed methane through the active area.

“I was taking out the stretchers to the street, they were coming out asleep without a single scratch,” he said.

“It was simply the methane displacing the oxygen, an overwhelming asphyxiation. One mouthful of that and that's it, like birds.”

He said that most of the victims were in their 30s with families, and another one nearing retirement.

The work halt for all Spanish operations was called by major Spanish union groups the Comisiones Obreras (CCOO) and the Metal, Construction and Allied Workers' Federation (UGT).

“[We] have decided to call a 48-hour strike in all mining operations in the country as a symbol of respect, condolences and mourning for our workmates,” they said in a joint statement, while urging the National Commission for Mining Safety to investigate the incident and take needed measures to prevent other disasters.

The mine’s owner, Hullera Vasco Leonesa, the AFP said, blamed a methane gas leak and said an internal investigation has started.

The incident is the worst accident at a Spanish mine since 14 people were killed in a methane explosion near Mieres in August 1995.

About 40 mines are in operation in Spain. Most of those are in the northern region.

According to national data, the aim is to lower employment from about 8000 in 2011 to 4500 by 2018.