A miner stands next to machinery at a mine in this still image taken from video at Grey District, New Zealand, November 19, 2010. [Photo/Agencies]

But unlike the accident in Chile, where 33 men were rescued from a gold and copper mine after being trapped a half mile (one kilometer) underground for 69 days, Pike River officials have to worry about the presence of methane, mine safety expert David Feickert said.

He added, however, that the Pike River mine has two exits, while the mine in Chile had only one access shaft that was blocked.

The coal seam at the mine is reached through a 1.4-mile (2.3-kilometer) horizontal tunnel into the mountain. The seam lies about 650 feet (200 meters) beneath the surface. The vertical ventilation shaft rises 354 feet (108 meters) from the tunnel to the surface.

Whittall said the horizontal tunnel would make any rescue easier than a steep-angled shaft, once safety is established.

Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for several days.

But some said hope was fading for the men.

"The longer it drags on it doesn't look good, does it?" said local resident Shayne Gregg, who worked at the mine last year. "It's a feeling of hopelessness not ... being able to get there, but people are aware the mining industry is hazardous and has highs and lows."

Australian and British citizens were among the missing men, and Australia sent a team of mine rescue experts to assist the operation.

Australian Prime Minister Julia Gillard said Australia was anxiously waiting for news from the mine. British Foreign Secretary William Hague said he was saddened by the accident.

The 2-year-old Pike River mine is working largest-known deposit of hard coking coal in New Zealand, with 58.5 million tons.

New Zealand has a generally safe mining sector, with 181 people killed in 114 years. The worst disaster was in March 1896, when 65 died in a gas explosion. Friday's explosion occurred in the same coal seam.