Saman Gunan, 38, lost consciousness on his way out of the Tham Luang cave complex after delivering supplies and could not be revived, reports the BBC.

"His job was to deliver oxygen. He did not have enough on his way back," said an official.

The diver had left the navy but returned to help the rescue operation.

"A former SEAL who volunteered to help died at around 2 a.m.," Chiang Rai Deputy Governor Passakorn Boonyaluck told reporters at the rescue site.

Gunan, said to be an avid runner and cyclist, was part of the massive ongoing operation which to rescue the boys, aged between 11 and 16, and their 25-year-old coach.

The boys went missing on June 23 after they had entered the cave in the Chiang Rai region during fine weather but became trapped when a sudden downpour flooded the narrow tunnels.

Around 1,000 people are involved in the rescue operations, including navy divers, military personnel and civilian volunteers, the BBC reported.

They were found on Monday night on a rock shelf about 4 km from the mouth of the cave by two British rescuer divers.

The cave complex is regularly flooded during the monsoon season which lasts until September or October.