Toxic gas is suspected of killing at least six people who died while exploring man-made tunnels in Spain's Canary Islands, officials said on Sunday.

A group of about 30 scientists and adventurers were roughly 1.5 kilometres underground on Saturday when they apparently were overcome by the gases, said Jose Miguel Ruano, Tenerife's regional government minister.

Six people were pulled from the tunnel dead, and six others were taken to hospital by helicopter, Ruano said.

Rescue efforts were made difficult because of the gases and cramped conditions underground, he said.

Officials said they believed members of the group fell unconscious when they were deprived of oxygen, but they didn't identify what kind of gases may have been involved.

Among the group were scientists from the Canary Island Astrophysics Institute and members of the Tenerife Friends of Nature Association.

The tunnels were built to prospect for water in the community of Los Silos on Tenerife, the largest of the Canary Islands.

With files from the Associated Press