The special hydraulic bore which will be used to drill an escape shaft down to 33 miners trapped in a Chilean mine is being assembled at the site.

"We've finished building the machine's platform... we hope between Sunday and Monday to begin drilling the shaft," chief engineer Andre Sougarret said.

The Strata 950 will drill a 60-70cm-wide shaft down which a capsule can be lowered to rescue the men one by one.

But even drilling round the clock, the rescue will probably take months.

On Friday Chile's Health Minister Jaime Manalich said five of the 33 miners, trapped 700m (2,300ft) below ground, were showing signs of depression.

They are reported to not be eating properly and refused to appear in a 45-minute video which the miners had filmed for their families.

"They are very isolated, they did not want to appear on the film, they are not eating well," Manalich said. "I would say depression is the right word."

He said psychologists would attempt to treat the men from the surface over an intercom system.

The men, who have already been stuck in the shaft for three weeks, were only discovered on 22 August.

Reports from the San José gold and copper mine, near the city of Copiapo, some 725km (450 miles) north of Santiago, say the men's relatives are being urged to write to their loved ones as often as possible as part of the effort to keep their spirits up.

The miners were told on Wednesday that it could take up to four months to rescue them.

At the time, Manalich said they had reacted calmly to the news, but he pointed out that they were "going to suffer from huge challenges regarding their psychological conditions".

A special exercise and recreation programme is being set up to keep the men mentally and physically fit during their long wait. They have also been told to use lighting to distinguish between day and night.

This week doctors from the US space agency Nasa, experts in keeping astronauts alive and well on long missions in confined spaces, will arrive in Chile to assist medical officials with the miners.

Despite their ordeal, most of the men looked relatively upbeat in a film broadcast on Chilean TV on Thursday.

A small tunnel has been drilled down to the men from the surface to allow supplies to be sent down.