Four of the 12 Thai schoolboys trapped in a flooded cave have been rescued and taken to hospital, a local official confirmed on Sunday, after the nation began a daring rescue operation.

A senior member of the rescue operation’s medical team said the four boys were rescued during the operation on Sunday, according to Reuters. The children have been safely transferred to hospital.

An additional eight boys and their football coach remain in the cave in Thailand's northern Chiang Rai province. A total of 90 divers – 50 foreign and 40 Thai – are involved in the rescue mission.

As night fell, the operation to save the remaining trapped boys and their coach was called off until Monday morning.

"Today we are at peak readiness," the lead rescuer said earlier on Sunday as he announced the start of the operation. The entire operation is expected to take three to four days, according to a military commander involved in the effort.

Thirteen medical teams have been stationed outside the cave, each of which has their own helicopter and ambulance.

Rescuers have been racing against the weather ever since they discovered the stranded football squad on Monday after they had already spent nine days underground. Monsoon storm clouds could bring rain at any moment, which would further flood the cave where the 12 boys, aged 11 to 16, and their 25-year-old coach have been cut off by rising water.

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Flooded underground passages, some only 0.6 meters (two feet) wide, separate the cave from the surface. Concerns were earlier voiced that the children, weakened by nine days without food, were unfit for diving all the way through.

Earlier, the children sent heart-wrenching letters to their families on the surface, and the coach apologized to their parents. Rescuers have been taking food to the boys and pumping air into the isolated cave.

The rescue effort, which has captivated the world throughout the week, was overshadowed on Thursday by the death of a 38-year-old former Marine, who died from a lack of oxygen after falling unconscious while on a supply run for the trapped boys.

The rescuers apparently chose not to wait for Elon Musk's "kid-sized submarine," which he supposedly started building on Saturday to ship to Thailand within a day. Musk said building the sub would only take eight hours. Counting from the time of Musk's tweet, the vessel was completed some two hours before the operation kicked off, but it would take another 17 hours to transport it. However, some commenters doubted the announced plan was anything more than a PR stunt on Musk's part.

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