Chile’s trapped miners say they are enduring “hell” underground, putting urgency into a rescue operation that is about to start but could drag on for months before providing salvation.

The 33 men, living in unimaginable conditions deep below ground for 20 days, pleaded with President Sebastian Pinera late Tuesday to save them, in an exchange over an intercom line dropped through a narrow drill hole.

“We are waiting for all of Chile to do everything to get us out of this hell,” said group leader Luis Urzua.

Mr Urzua added that all the miners were in good condition physically and had high hopes of rescue after finally being located last Sunday by a probe attached to the end of a drill bit.

Entombed for nearly three weeks 700 metres deep in a mine that collapsed in northern Chile August 5, the men had survived on meagre rations and trickles of water.

They were holed up in a reinforced shelter roughly the size of a living room, and were able to roam a lengthy tunnel.

Although the drill hole was now providing them with sustenance and water, and a line of communication to waiting families, officials were worried how they would hold up once they knew they would be prisoners of the earth for perhaps the rest of the year.

That despairing news was being withheld from the miners out of concern for their mental health.

In the meantime, preparations were being made to start a hydraulic bore that would grind a shaft down to the miners big enough to eventually extract them one by one.

The engineer in charge of the rescue mission, Andres Sougarret, told reporters the heavy machine should begin its long task “by the end of the week,” after a route was worked out that would not trigger another collapse.

In the intercom conversation, Mr Urzua urged rescuers to hurry, saying the men were hoping to be out by September 18, when Chile celebrates its Independence Day.

“Mr President,” he said, “we need you to be strong and to rescue us as soon as possible. Don’t abandon us.”

“You won’t be left alone, nor have you been alone a single moment,” Mr Pinera answered – without letting on how long the rescue operation would really take.

“The government is with you, the whole country is with you and I want you to stay calm because your families are cared for and have all our support,” the president added.

Chilean officials said they had asked the US space agency Nasa for help in supplying the miners with nourishment.

“The situation is very similar to the one experienced by astronauts who spend months on end in the space station,” Health Minister Jaime Manalich told reporters.

Nasa sent an email to AFP saying it had received a request from Chile and was “prepared to provide such support as requested.”

The Health Minister said two of the trapped miners had nursing experience, which will be a “tremendous asset” should medical expertise be required.

The miners, whose survival triggered a wave of national euphoria Sunday, said they were hungry after surviving on emergency rations of two tablespoons of tuna fish and half a cup of milk every 48 hours.

The miners range in age from 19 to 63. Many were football fans, Chilean media reported, and at least one had diabetes.

Paula Newman, a doctor in charge of monitoring their health, said glucose solution and medication had been sent down to prevent ulcers. Later, they were to be given high-protein, high-calorie foods.

“They are all in perfect health, and none are traumatised,” said Dr Newman. “Their complaints are much less than we could have expected.”