Riot police fire plastic paintball gun pellets to disperse protesters during a demonstration blaming the ruling AK Party (AKP) government for the mining disaster in western Turkey, in central Istanbul May 14, 2014. [Photo/Agencies]

Fire knocked out power at the mine and shut down ventilation shafts and elevators on Tuesday afternoon. Emergency workers pumped oxygen into the mine to try to keep those trapped alive during a rescue effort that lasted through the night. Thousands of family members and co-workers gathered outside the town's hospital searching for information on their loved ones.

"We haven't heard anything from any of them, not among the injured, not among the list of dead," said one elderly woman, Sengul, whose two nephews worked in the mine along with the sons of two of her neighbours.

"It's what people do here, risking their lives for two cents ... They say one gallery in the mine has not been reached, but it's almost been a day," she said.

A mechanical digger opened a row of fresh graves at Soma's main cemetery. An imam presided over the funeral of six miners as a few hundred mourners wept in silence.

274 dead in Turkey's worst-ever mine disaster

The fire broke out during a shift change, leading to uncertainty over the exact number of miners trapped. Energy Minister Taner Yildiz said the death toll as of 1930 GMT on Wednesday was 274, making it Turkey's deadliest accident. Late on Tuesday he said 787 workers had been in the mine.

Yildiz warned that "hopes were dimming" of finding many more survivors.

Turkey's safety record in coal mining has been poor for decades, with its previous worst accident in 1992, when a gas blast killed 263 workers in the Black Sea province of Zonguldak.

The mine operator Soma Komur Isletmeleri said nearly 450 miners had been rescued and that the deaths were caused by carbon monoxide. It said the cause was not yet clear.

Initial reports suggested an electrical fault caused the blaze but Mehmet Torun, a board member and former head of the Chamber of Mining Engineers who was at the scene, said a disused coal seam had heated up, expelling carbon monoxide through the mine's tunnels and galleries.

"They are ventilating the shafts but carbon monoxide kills in 3 or 5 minutes," he told by telephone.

"Unless we have a major miracle, we shouldn't expect anyone to emerge alive at this point," he said, pointing to an outside chance that workers may have found air pockets to survive.