(Updates with fresh President Pinera quote, plan details)

By Alonso Soto

COPIAPO, Chile (Reuters) - Rescuers readied
Sunday to drill small holes deep inside a mine in northern
Chile to try to locate 33 miners trapped for three days by
cave-ins and keep them alive after a bid to reach them failed.

The rescue effort faced a major setback Saturday when a
fresh collapse blocked the ventilation shaft rescue workers
were descending. No contact has yet been made with the miners,
trapped since Thursday 4.5 miles inside the winding
mine, and around 900 feet vertically underground.

Authorities hope they managed to reach an underground
shelter with oxygen and are rationing food and water.

The hope is now to find them and pass them fresh supplies
of water and food and communication lines down a narrow drill
hole until experts find a way to extract them from the small
gold and copper mine.

Workers used mechanical diggers to clear a path for a
drilling rig to enter the mine to start the perforations, and
were also trying to clear the cave-ins along the main mine ramp
underground and the ventilation shaft.

"I personally promised the families of the trapped miners
the rescue operations will continue with all the strength in
the world," President Sebastian Pinera said Sunday at his
palace in Santiago, after cutting short a visit to Colombia to
fly home to meet families at the mine late Saturday.

"Unfortunately it is not easy," he added. "I also pledged
to thoroughly investigate, establish where responsibility lies,
and punish accordingly."

Rescuers face a race against time to reach the miners, who,
if they reached the shelter, are huddled in a space the size of
a small apartment.

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SCENARIOS-Mine accident spotlights safety [ID:nN06239625]

FACTBOX-Major Chile mine accidents [ID:nN06266599]

For a graphic, see http://link.reuters.com/baj73n

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The San Jose mine, which belongs to local private company
Compania Minera San Esteban Primera, was shut down by the
accident. It is one of three adjacent sites producing a
combined 1,200 metric tonnes of refined copper annually.

The closure is not expected to hurt copper output in Chile,
the world's No.1 copper producer. Major mining accidents are
uncommon in the country, because authorities keep strict
controls over operations.

Anguished relatives, who spent a second night at the mouth
of the mine around a bonfire, wailed when they heard news of
the fresh collapse Saturday.

They later calmed down when they were told the mine had not
collapsed entirely and rescue efforts would continue, though
several then tried to storm the mine entrance and scuffled
briefly with police, calling company officials "murderers".

"We have to wait because the rescue will go on," said local
trader Crisologo Rojas, 51, whose nephew and friends are
trapped in the mine, 450 miles north of the capital,
Santiago. "I don't know what they're going to find down
there."

Local union official Felix Medina said conditions at many
smaller mines in the area are precarious. He said the mine,
which employs 150 people, has a recent history of accidents,
with 13 fatalities on site and three deaths on the winding road
to Copiapo.

The mine lacks escape routes and was closed in 2005 by
workers because of bad conditions, he added. It was reopened
two years ago.
(Writing by Simon Gardner, Editing by Sandra Maler)