(Recasts with rescuers refusing to give up hope)

By James Nelson

HUNTINGTON, Utah, Aug 10 (Reuters) - Rescuers trying to reach six men trapped in a collapsed Utah coal mine refused to give up hope of finding them alive on Friday, even though instruments dangled into an underground chamber found no signs of life.

The six miners have not been heard from in five days, since the Crandall Canyon Mine near Huntington caved in on Monday morning, and did not respond when a microphone was lowered 1,800 feet (550 meters) through a drill hole.

The rescuers said it was possible that they had missed the shaft where the miners were thought trapped and drilled instead into a sealed chamber nearby.

Initial tests taken through the 2-1/2-inch (5-cm) bore hole of the air in the mine showed it could sustain life but later readings indicated much lower oxygen levels.

"There is no reason to lose hope," Richard Stickler, head of the Mine Safety and Health Administration, told reporters at a news conference. "There is certainly a possibility these miners are still alive, because we don't know where this bore hole drilled in."

"At this point, the thing to do is continue on our plan, maintain our hope," Stickler said.

Crews have removed the microphone and will lower a survey instrument to pinpoint the drill hole's location in the mine, Stickler said.

'WE WON'T STOP'

A second, 9-inch (23-cm) hole that will allow rescuers to lower a camera into the mine -- and could provide a way to give the miners air, food and water if they are still alive -- is still being drilled but officials would not say when they expected it to be completed.

Officials earlier had said they expected the larger hole to punch through into the mine late on Friday or early on Saturday.

Rob Moore, vice president of mine co-owner Murray Energy, said directional devices used on the 9-inch drill gave it a better chance of hitting its target.

Robert Murray, chairman of Murray Energy, said earlier on Friday that it would take at least another four to five days to clear the fallen rock and coal and reach the miners with an opening large enough to pull them to safety.

Officials say the men could potentially survive for weeks in an underground chamber if they were not killed by the initial collapse.

"We won't stop until we get them out," said Allan Borba, a a 39-year-old former miner whose cousin, Kerry Allred, was one of the men stranded underground.

Borba, whose foot was amputated at the ankle in a 1990 mining accident, has not been allowed to participate in the rescue effort but has rallied support for the men.

"It bothers me that I can't put a boot on right now and go help," he said.

Murray has insisted that an earthquake triggered the mine's collapse but geologists dispute that, saying that shaking recorded by their instruments was caused by the cave-in.

Controversy has also risen over reports that the miners were engaged in a dangerous operation called "retreat mining" when the shaft collapsed -- though Murray has denied that such a technique was being used.

Retreat mining involves supporting the mine's roof with a column of coal, then removing those pillars and allowing the shaft to collapse as miners move to safety.

The Crandall Canyon Mine is on a high desert plateau some 140 miles (225 km) south of Salt Lake City, in what is known as Utah's "castle country" because of the towering rock spires that dot the bleak landscape. (Additional reporting by Dan Whitcomb in Los Angeles)

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