CHIANG RAI, Thailand, July 8 (Xinhua) -- Rescue operation to bring out 12 boys and their coach trapped in northern Thailand's cave has been underway since about 10:00 a.m. local time on Sunday, Chiang Rai governor Narongsak Osottanakom confirmed at a briefing.

He said 13 international cave diving experts and five elite rescuers from Thai Navy Seal joined the operation, adding that the trapped boys will be dived out one by one.

Each boy will be escorted by two rescuers, he said.

The governor said it would take about 11 hours to totally bring a boy out of the cave. The first boy may come out of the cave at about 9 p.m. local time.

He said it will take about three to four days to bring all the 13 out of the cave.

The boys, ages 11-16, and their 25-year-old coach went missing in the cave since June 23. Rescue teams launched a round-the-clock search and found them on July 2.

A former Thai navy SEAL has died from lack of oxygen during the rescue operation on Friday.