Officials in Chile are working on an alternative plan to speed the rescue of 33 miners trapped underground since Aug. 5.

Work on drilling an escape shaft about 700 metres underground is set to begin Monday at the San Jose mine, a task that could take four months to complete.

A video image shows a trapped miner waving at the camera. (Television Nacional de Chile/Associated Press)

But engineers say an existing tunnel to the men could be widened in two months.

The miners have been trapped under the Atacama Desert, 700 kilometres north of Santiago, since a tunnel collapse at the mine three weeks ago and were only discovered alive last Sunday.

Health Minister Jaime Manalich announced on Saturday that "Plan B has already been designed," promising more details later.

However, Mining Minister Laurence Golborne had earlier dismissed speculation the miners could be rescued in as little as 30 or 60 days.

"The information is extremely clear, the time frame is … between three and four months," Mr Golborne told local media, according to AFP news agency.

Video footage of the miners taken in their emergency refuge and broadcast on Chilean television late Thursday shows most of the men in relatively good spirits.

The camera used to make the video was sent down through a bore-hole used for communications. Another small hole that snakes down to the men's shelter is used for lowering food and a third provides ventilation.