A fourth explosion in nine days ripped through a New Zealand mine where 29 miners perished, and officials said on Sunday the coal was on fire, a development that could significantly delay recovery of the bodies.

Large quantities of smoke and flames were seen shooting from the Pike River Coal mine’s vertical ventilation shaft after Sunday’s blast.

“This smoke has changed, it’s no longer a gas fire, it’s obviously now a coal fire,” Pike River chief executive Peter Whittall told reporters. “Where that coal fire is or how big it is, we don’t know.”

The mine might have to be temporarily sealed to starve the fire of oxygen, Mr. Whittall said, without indicating how long it would take to kill the fire. The move could seriously delay the recovery of the bodies, and Mr. Whittall said it was not the preferred option.

The explosions have dislodged a lot of coal, “so there’s a lot of fuel in the mine to burn,” he said.

The worst-case scenario was that the actual coal seam would start to burn, he said. A gas fire is relatively easy to put out, but a coal fire in a seam would be a “very different beast,” Mr. Whittall said.

Operators still hope to deploy an Australian jet-powered engine to blast nitrogen and carbon dioxide gases and water vapour into the mine. The inert gases would expel oxygen that could fuel more explosions, and would smother the fire. That option may be ready to use on Monday.

There were no injuries from Sunday’s blast, and a brief fire involving vegetation around the surface vent was extinguished. People working near the mine entrance were moved away from the area for safety.

The 29 miners were trapped by the first blast on November 19 and declared dead after a massive second blast five days later. A third explosion on Friday was fuelled by methane gas seeping into the mine.

Police superintendent Dave Cliff said the latest explosion demonstrates the volatility of the mine environment, which has prevented any rescue workers from entering the mine since the first blast.

“We are doing all we can to progress the recovery operation, however the explosion reinforces the risks involved in working in this environment and the requirement to put people’s safety first,” he told reporters.

Prime Minister John Key said Sunday that a Royal Commission of Inquiry will be held into the disaster — showing that the government was taking the inquiry seriously and was determined to find answers for the dead miners’ families.

Ten busloads of families were taken to the site on Saturday, but none of the relatives has publicly commented since then.

New Zealand’s mining industry is small and generally considered safe. The tragedy deeply shocked the country and devastated families who — buoyed by the survival tale of Chile’s 33 buried miners — had clung to hope that their relatives could emerge alive.

The country has had 210 deaths in 114 years in mines. New Zealand’s worst mine disaster was in 1896, when 65 died in a gas explosion at a mine on the same coal seam as the latest tragedy.

The most recent had been in 1967, when an explosion killed 19 miners near the Pike River site. A fire in a mine in 1914 killed 43.

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