Wellington, New Zealand - Search efforts for 29 workers missing after a powerful blast tore through a coal mine in New Zealand were stalled today over fears that a buildup of dangerous gas could trigger a second underground explosion.

Repeated attempts to contact the 16 employees and 13 contract miners after Friday's explosion at the Pike River mine have failed, and it was not known if the men were still alive, Pike River Mine Ltd's chief executive Peter Whittall said. The blast was most likely caused by coal gas igniting, he said.

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.

Search controller, police superintendent Gary Knowles, said they were still waiting for air test results from the mine before search teams can be sent in.

"As the search commander I'm not prepared to put people on the ground until we can be sure it's a safe environment," he said.

Whittall said the hazard for rescuers was the danger of something sparking a gas explosion.

It could be days before it is safe enough for special teams to enter the mine, said Tony Kokshoorn, mayor of nearby Greymouth.

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.

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

Unlike the mine accident in Chile where 33 men were rescued from a gold and copper mine last month 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.

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 today to pump in fresh air and make it safe for rescuers.

Kokshoorn was hopeful the missing workers could survive like the Chilean miners, whose rescue - played out on live television - captivated the world.

"We are holding on to hope," Kokshoorn said. "Look at Chile, all those miners were trapped and they all came out alive."

He said his community was "grief-stricken" about the missing miners and had "ground to a halt" as they wait for news about the men.

"The families out there ... are all rallying together at the moment, we're doing everything we can for them but it's a serious situation down there," he told TV3's "The Nation" program.

"We're praying for them," he said.

Australian and British nationals were among the missing men, Energy Minister Gerry Brownlee said.

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.

Deputy Mayor Doug Truman said the blast was so powerful that one of the workers who came out of the mine described being only a mile inside the shaft when he was blown off his machine.

"The mine vents have ... scorch marks - so it must have been a reasonably big explosion," Truman told New Zealand's National Radio.

Brownlee said the explosion occurred about 3:45 p.m. (0245 GMT Friday, 9:45 p.m. EST Thursday), 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, Whittall said.

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.

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.

It said its coal preparation plant at the site is the largest and most modern in New Zealand and processes up to 1.5 million tons of raw coal a year. It is country's largest single source of coal exports.

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.