WELLINGTON, New Zealand » Rescuers waited Saturday for gas levels and the risk of a second explosion to drop so they could enter New Zealand’s biggest underground coal mine to search for 29 men still missing more than 24 hours after a powerful blast tore through the tunnel.

It was not known if the 16 employees and 13 contract miners survived Friday’s explosion. Repeated attempts to contact them have been fruitless, Pike River Mine Ltd’s chief executive Peter Whittall said.

"We haven’t heard a thing," Whittall said.

A working phone line to the bottom of the mine had rung unanswered. Whittall said one of two men who escaped from the mine had used the phone to contact the surface before walking out.

The two dazed and slightly injured miners stumbled to the surface hours after the blast shot up the mine’s 354-foot-long ventilation shaft. 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 New Zealand’s South Island.

The blast was most likely caused by coal gas igniting, Whittall said. The extent of damage underground was unknown.

The rescue effort has been delayed by more than 24 hours due to fears that a buildup of dangerous gases could trigger a second underground explosion. The police search controller, superintendent Gary Knowles, said rescuers were ready to go as soon as air quality tests showed low enough gas levels for a safe search.

"We’re not going to put 16 men underground and risk their lives," he told reporters.

He remained confident that the missing miners were alive.

"This is a search and rescue operation, and we are going to bring these guys home," Knowles said.

Families of the missing men gathered at a Red Cross hall in nearby Greymouth on Saturday, and were being briefed hourly as the rescue drama unfolded. They have declined to talk to reporters, as have the two men who made it out of the mine.

Air samples taken from the mine were being analyzed for traces of methane, carbon monoxide, ethane and other trace gases, and authorities especially wanted to see lowering levels of carbon monoxide, but that had not happened yet, Whittall said.

Whittall said a compressed air line damaged in the explosion was still pumping fresh air into the mine and was "flowing very freely."

"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.

Electricity went out shortly before Friday’s explosion and that failure may have caused ventilation problems and contributed to a buildup of gas. The power outage continued to frustrate efforts Saturday to pump in fresh air and make it safe for rescuers.

The missing miners would have to deal with numerous hazards, including air pollution, high levels of methane and carbon dioxide, and low levels of oxygen, 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 nationals were among the missing men, Energy Minister Gerry Brownlee said.

Australia is sending a team of mine rescue experts to New Zealand to assist the rescue efforts, Australian Foreign Minister Kevin Rudd said.

"What mates do is we help when things go wrong and that’s what Australia is doing and we’ll be there to support our Kiwi friends in a very difficult hour," Rudd said.

Unlike the recent mine accident in Chile where 33 men were rescued from a gold and copper mine after being trapped a half-mile 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 horizontal tunnel into the mountain. The seam lies about 650 feet beneath the surface. According to the company’s website, the vertical ventilation shaft rises 354 feet 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.

Brownlee said the explosion occurred about 3:45 p.m. Friday, and the last contact with any of the miners was about a half-hour later. That contact was with one of the two men who came out.

The two men who surfaced were taken to a hospital for treatment of minor injuries and were being interviewed to determine what happened.

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.