WELLINGTON, New Zealand -- Rescuers prepared to drill a gas sampling hole through 500 feet (150 meters) of hard rock and into a New Zealand coal mine Sunday where a powerful blast trapped 29 workers nearly two days ago.

Pike River Mine Ltd. chief executive Peter Whittall told reporters a small six inch(15 centimeter)-wide hole would be drilled into the mine from the mountain above over the next 16 to 24 hours to enable rescuers to sample gas levels from deep in the mine's center.

Underground combustion that continues to generate dangerous gases was preventing rescuers from entering the mine.

"We've got a heating of some sort underground and that means there's some combustion generating the gases that go with that, carbon monoxide, a slight increase in methane and some other gases," Whitehall said. "Something is happening underground, but what it is we don't know."

The small six-inch (15 centimeter) hole, to be drilled through the mountainside, was aimed at an area a couple of hundred meters (660 feet) further into the mine from where the main ventilation shaft is located.

"That will give us more information about what's going on" with gas levels near the center of the mine, he said.

Dangerous methane and carbon monoxide gas levels meant "it's still not safe for rescue teams to enter the mine," Whittall said, though "the oxygen quality coming out of the mine is still high."

Fresh air is still being pumped into the mine through an open air line.

Police search controller, superintendent Gary Knowles, said the "primary focus today (Sunday) is still a rescue operation.

"Our major focus is bringing these guys home," he told reporters, "but we need a stable air environment. When we have a stable environment ... and as soon as we have that we're going in."

Anguished relatives voiced frustration over the delays in the rescue operation.

"If I had my way I'd be down there, I'd go into the mine myself," said Laurie Drew, whose 21-year-old son, Zen, is one of the missing men.

Police said the miners, aged 17 to 62, are believed to be about 1.2 miles (two kilometers) down the main tunnel.

Two men emerged after the explosion Friday, but there has been no word from the 29 others.

Electricity in the mine went out shortly before the explosion and that failure may have caused ventilation problems and contributed to a buildup of gas.

Whittall noted the blast was most likely caused by coal gas igniting.

The power outage continued to frustrate efforts Saturday to pump in fresh air and make it safe for rescuers, though Whittall said air was flowing freely through a compressed air line damaged in the explosion.

"We have kept those compressors going and we are pumping fresh air into the mine somewhere. It is quite conceivable there is a large number of men sitting around the end of that open pipe waiting and wondering why we are taking our time getting to them," Whittall said.

A working phone line to the bottom of the mine, however, had rung unanswered.

The two dazed and slightly injured miners stumbled to the surface hours after the blast shot up the mine's 354-foot (108-meter)-long ventilation shaft. The men were taken to a hospital for treatment of minor injuries and were being interviewed to determine what happened. Whittall said one of the two men had used the phone to contact the surface before walking out.

The explosion occurred about 3:45 p.m. Friday. Video from the scene showed blackened trees and light smoke billowing from the top of the rugged mountain where the mine is located, near Atarau on South Island. It is New Zealand's largest underground coal mine.

Families of the missing men gathered at a Red Cross hall in nearby Greymouth on Saturday, and were being briefed hourly on rescue efforts. Most have declined to talk to reporters, as have the two men who made it out of the mine. They are expected to be taken to the mine site on Sunday.

Knowles said the families "are pretty distraught ... but they understand the risks we're facing."

After visiting the families, Prime Minister John Key told reporters that "there is a great sense of anxiety and genuine fear, and I think that's only natural given the ... difficulty of the situation."

"We reflected to them that they have to hang on to hope. As we saw in the case of the Chilean mine, 33 miners did get out alive," he said.

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. According to the company's website, 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.

"We're not a deep-shafted mine so men and rescue teams can get in and out quite effectively, and they'll be able to explore the mine quite quickly," he said.

Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for "several days," said Pike River chairman John Dow.

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

"We know our New Zealand colleagues are doing everything they can to effect a rescue," Australian Prime Minister Julia Gillard said in Lisbon, where she was attending a NATO summit. "We're waiting for further news but it could be some time yet."

British Foreign Secretary William Hague said he was saddened by the accident.

"My thoughts are with those who are missing, and also with their families and friends, who are awaiting news," he said in a statement.

While Pike River Coal is a New Zealand-registered company, its majority owners are Australian. There are also Indian shareholders.

Pike River has operated since 2008, mining a seam with 58.5 million tons of coal, the largest-known deposit of hard coking coal in New Zealand, according to its website.

The mine is not far from the site of one of New Zealand's worst mining disasters -- an underground explosion in the state-owned Strongman Mine on Jan. 19, 1967, that killed 19 workers.

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.

"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."

But father Laurie Drew said he was frustrated by the lack of action from rescuers, who he said were giving excuses instead of finding solutions.

As he spoke to TV One, Drew wore his son's jacket. "I wore it so I can give it back to him when he comes out," he said, choking back tears. "I just want my boy home."