Efforts to find six miners trapped deep underground after a Utah mine collapse wore into a ninth day on Tuesday, still with no sign of the missing men.

Rescuers were drilling a third hole into the Crandall Canyon Mine in central Utah, hoping to find the men in an area of the mine where they might have retreated to find air pockets after Monday's cave-in.

Two holes drilled into the mine earlier this week failed to turn up any sign of the miners, though a camera dangled into an underground chamber showed tool bags and equipment possibly belonging to them.

"We're pressing as fast as we can to rescue the trapped miners," mine co-owner Robert Murray told reporters. "We will spare nothing in our efforts to get to the trapped miners underground."

Crews were also burrowing a tunnel into the mine that would allow them to pull the men out if they are found alive, but that work has gone slowly due to what Murray has described as "seismic activity" underground.

So far workers have dug about 700 feet (213 metres) of that tunnel and had another 1,200 feet (365 metres) to go, or another five to seven days of work without other delays.

Officials concede that the men could already be dead, crushed more than 1,800 feet (550 metres) below the surface when the mine caved in for reasons that have yet to be determined.

Murray has insisted that the mine collapse was triggered by an earthquake despite experts' assertions to the contrary.

About a dozen miners working on the large escape tunnel had been reassigned after expressing fears for their own safety.