MAE SAI, Thailand -- The soccer coach trapped in a cave with 12 Thai boys has apologized to their parents in the first letter he and the team have sent out through divers. The boys wrote letters to their parents saying "don't worry ... we are strong," BBC News reports.

The 25-year-old coach says: "To the parents of all the kids, right now the kids are all fine, the crew are taking good care. I promise I will care for the kids as best as possible. I want to say thanks for all the support and I want to apologize to the parents."

Rescuers say they won't immediately attempt an underwater evacuation because the boys have not yet learned adequate diving skills. But if heavy rains start again, divers will try to take the boys out right away.

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The boys also wrote they are doing well and missing their families. The group's handwritten notes include requests for different types of food.

"Teacher, don't give us lots of homework!" read one.

Narongsak Osotthanakorn, governor of the Chiang Rai region, said Friday the health of most of the boys had "improved to normal," according to BBC News.

Narongsak, who is in immediate charge of the operation, indicated at a news conference, however, that if heavy rains started and appeared to be causing flooded areas in the cave to rise again, divers would try to take the boys out right away.

Thai officials had been suggesting in public statements that a quick underwater evacuation of the boys and their soccer coach was needed because of the possibility that access to the cave could soon close again due to seasonal monsoon rains expected this weekend.

Cave rescue specialists have cautioned against that approach except as a last resort, because of the dangers posed by inexperienced people using diving gear. The path out is considered especially complicated because of twists and turns in some narrow flooded passages.

Earlier Friday, Thai SEAL commander Arpakorn Yookongkaew said there is a "limited amount of time" to rescue the boys and the coach.

"We can no longer wait for all conditions (to be ready) because circumstances are pressuring us," Thai SEAL commander Arpakorn Yookongkaew said at a news conference. "We originally thought the boys can stay safe inside the cave for quite some time but circumstances have changed."

A suggestion that the trapped team might have to wait months inside until a safe way out in available -- as was the case in 2010 with Chilean miners trapped underground -- has met with little enthusiasm. Authorities continue to pursue a third option, which is finding a shaft or drilling into the mountain in which the cave is located to find a sort of back door entrance.

The boys, 11 to 16, and their 25-year-old coach went exploring in the cave after a soccer game June 23. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days. The only way to reach them was by navigating dark and tight passageways filled with muddy water and strong currents.

Asked at his midnight news conference about taking the boys out underwater, the governor replied, "Not today because they cannot dive at this time."

Narongsak said the boys were still healthy and have practiced wearing diving masks and breathing in preparation for the diving possibility. According to BBC News, teams of Thai and international divers have been supplying the boys and the coach with food, oxygen and medical attention. But there are mounting concerns about the oxygen level in the chamber, which officials said had fallen to 15 percent from the usual level of 21 percent.

With the oxygen levels inside getting lower as more workers enter, authorities on Friday were trying to install a 3-mile-long oxygen tube to pump in air, CBS News correspondent Ben Tracy reported.

The rescue effort suffered a disheartening setback Friday with the death of a former Thai Navy SEAL diving in the flooded passageways to deliver supplies, as authorities raced against worsening weather and lessening oxygen.