There is "every chance" the 29 men missing since Friday's explosion at a coal mine in New Zealand are still alive, Prime Minister John Key says.
There has been no contact with the men since the blast at the Pike River mine near Greymouth on the South Island.
Despite their optimism, officials are preparing for all eventualities, including the "possible loss of life".
A 15cm (6in) wide shaft is being drilled into the mine, to check gas levels and listen for signs of life.
A military robot equipped with cameras will also be sent into the area where the miners are believed to be trapped.
The remotely-operated vehicle is used to perform reconnaissance, object retrieval and weapons delivery in situations hazardous to humans.
At a news conference, Mr Key said the miners - who include 24 New Zealanders, two Australians, two Britons and a South African - might have found refuge in a ventilated spot and could still be rescued.
"The advice I have is that there is oxygen in the mine and there is every chance that those miners have managed to get to a pocket of that oxygen flow and therefore that they are alive," he said.
The prime minister's determination will give comfort to the families and friends of the missing men, says the BBC's Phil Mercer in Greymouth.
For three days they have waited for the slightest sign that the men are alive, but there has been only silence, our correspondent adds.
Police Supt Gary Knowles, who is in charge of the rescue operation, said officials were nevertheless planning for "all outcomes", including the "possible loss of life as a result of what's occurred underground".
But he was still optimistic, he told a news conference on Monday, and rescuers were "doing everything we can humanly do" to find the men.
The chief executive of the mine's operator, Pike River Coal, said the narrow borehole would be used to take gather pictures and audio from inside the mine.
"But most importantly we'll be able to take new gas samples and have them analysed straight away," Peter Whittall said.
Multiple tests would be needed before it could be established beyond reasonable doubt that conditions were safe to enter, officials said.
Officials have released the names of the miners.
The youngest, 17-year-old Joseph Dunbar, was reportedly on his first underground shift. The oldest is Keith Valli, aged 62.
Meanwhile, the mayor of Greymouth, Tony Kokshoorn, said locals felt as if they were "in limbo" as they waited to discover the miners' fate.
"Every hour and every day that goes by we have to face the facts that they can't stay down there indefinitely," he said.
"We've got to get some resolution, some movement forward, because the desperation is turning to grief."
Tearful family members have been taken on tours of the mine site to help them understand the dangers of an attempted rescue, and to show them the drills under way in preparation for an eventual operation.
Laurie Drew, whose 21-year-old son Zen is among those trapped, said it was hard seeing a board with the men's name tags missing.
"We have got faith that they are going to come out... but it's just how long is it going to take to find out for sure what's going on down there," he said. "We have just got to be strong for our loved ones and hope they come out. I hope they all walk out."
Gas samples from the mine's main ventilation shaft are still showing high levels of carbon dioxide and methane, which rescue officials say make it unsafe for crews underground.
Authorities are facing increasing pressure to allow rescue teams to enter the mine, but have repeatedly stressed the need to wait until it is safe.
Their great fear is that searching the mine could spark a second explosion.
Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for several days. Fresh air is also being pumped into the mine through a shaft, although it is likely to be hot and stuffy.
While the men would reportedly have been carrying flasks of water, there is no food underground. Their cap lamps would run out after about 24 hours, officials said.
Two workers who walked out of the mine have been been discharged from hospital after receiving treatment for moderate injuries.
One of them, coal cutter Russell Smith, said he had been late for work and so was not far into the mine when the explosion hit.
He saw a flash felt a series of shockwaves before he was knocked unconscious by the strength of the blast.
"My hat was... torn off me," he told New Zealand's TV3 network. "I remember struggling for breath."
He said he had been very lucky.
"I could have been blown to bits," he said.
He was found about 15m away from his vehicle, and together with Daniel Rockhouse found his way out of the mine.