More than 150 rescuers join effort to free ill American from Turkish cave

In video brought to surface, Mark Dickey says he was very close to the edge' after falling ill 1km down

Local and international cave rescue teams are poised to begin an attempt to evacuate a sick American caver from a deep underground system in Turkey, in what experts are suggesting may be one of the most complex operations of its kind.

Turkish-led rescuers have said the operation to bring up 40-year-old Mark Dickey by stretcher could take up to four days, and will require constant monitoring of his health.