WELLINGTON (Reuters) - Twenty-nine miners trapped in a remote New Zealand coal mine had still not made any contact with rescuers on Saturday, a day after an explosion ripped through the colliery, New Zealand authorities said.

The mine’s owner revised up the number of miners underground to 29, from initial reports of 27, as rescue teams prepared to enter the mine, waiting for an all-clear from experts checking for underground gases and assessing risks of another explosion.

“We have determined it was an explosion, most likely a methane explosion,” Pike River Coal chief executive Peter Whittall told a news conference in Greymouth, the nearest town to the mine dug into a mountainside on the rugged South Island’s west coast.

He said underground communications were down, except for one emergency underground phone, which rescuers had been calling constantly without anyone picking up. But he stressed the miners might be trapped in an area away from the phone, or unwilling to venture from an area of safety to answer it.

“We have not had any communications with the rest of the workforce underground,” Whittall said, but he hoped all the miners were safe and were sitting around one of the mine’s fresh air vents, waiting to be rescued.

“It’s quite conceivable that there is a large number of men sitting around the end of the open (ventilation) pipe, waiting and wondering why we are taking our time to get to them.”

It would take at least six to eight hours to assess air quality in the mine, Whittall added, setting up an agonizing wait for family and friends of the trapped group, which includes miners aged from their early 20s to one man aged 62.

The miners also include several Britons and Australians.

The accident follows the ordeal of 33 Chilean miners trapped in an underground chamber for two months before their dramatic rescue last month, when they were hoisted one by one to safety through a hole drilled 700 meters (2,300 feet) through rock.

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Whittall said the New Zealand mine gave rescuers an advantage over the Chilean situation, given the Pike River colliery’s main shaft was a horizontal tunnel, enabling easy access by foot or heavy vehicle.

The mine is about 2.3 kilometers (1.4 miles) long, with the trapped men believed to be most of the way in. There are a number of ventilation shafts which climb vertically at least 100 meters to the surface to provide fresh air to the mine.

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The possibility of gas within the mine is the biggest danger for both the trapped miners and rescuers. Methane gas is a common danger in coal-mining and is highly combustible. Carbon monoxide poisoning is also a risk.

“No one has been able to go underground at the moment because the risk to personnel is still too great,” Whittall said.

Police sought to give a morale-boost to family and friends of the trapped miners, vowing to get them all out safely.

“This is a search and rescue operation and we are going to bring these guys home,” district police commander Superintendent Gary Knowles told reporters at the news conference.

The miners each carry an emergency oxygen supply lasting only 30 minutes to an hour, enough to enable them to escape or find refuge in an emergency chamber linked to the ventilation shafts.

Two men escaped the mine in the moments after the blast on Friday afternoon, walking out of the main shaft, but they were unable to give rescuers the location of the others. One of the pair had called from the mine’s one functioning phone before walking out on his own.