A former Thai navy SEAL working to rescue boys trapped in cave in northern Thailand has died from lack of oxygen.

SEAL commander Arpakorn Yookongkaew told a news conference on Friday morning that the rescuer was working in a volunteer capacity and died during an overnight mission in which he was placing oxygen canisters.

"We are trained to expect risks at anytime," Arpakorn said. "It's part of the job."

Thai authorities are racing to pump out water from a flooded cave where 12 boys and their soccer coach have been trapped since June 23.

-AAP