After a short deluge of rain on Saturday night and with more bad weather forecast later on Sunday, Narongsak said authorities had to act immediately.
A heavy rain shower hit northern Thailand overnight, worsening conditions at the cave where rescuers were waging a "war with water and time".
"Today is the D-Day for the rescuers", said Narongsak Osottanakorn, the outgoing governor of Chiang Rai province.
Mission chief Narongsak said in recent days that medic teams had complained about the media presence and they told him "it will be a problem if they have a real emergency situation". "Finding the boys doesn't mean we've finished our mission". "We originally thought the boys can stay safe inside the cave for quite some time, but circumstances have changed".
'If we did not carry out the mission today we might not have been able to get them out.
A Thai governor says the operation to bring out 12 schoolboys and their soccer coach from deep inside a cave where they have been trapped for two weeks has begun.
The letters make for remarkable reading. Tell my brother to be ready to bring me fried chicken. Grandpa, Uncle, Mom, Dad and siblings, I love you all. They were found on Monday by Thai Navy SEALs and two British diving experts. "Night loves you all".
One wrote: "I'm doing fine, but the air is a little cold, but don't worry".
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The warning came a day after a Thai diver died during part of the rescue operation, marking a deadly turn in what started out as a celebration of one of the boys' birthdays at the Tham Luang cave complex in northern Chiang Rai province.
The football coach trapped in a cave with 12 Thai boys has apologised to their parents in a letter sent out through divers.
"To the parents of all the kids, right now the kids are all fine, the crew are taking good care". And take care of yourself. "I love you", wrote 11-year-old Chanintr Wiboonroongruang (Titan). "Also, air bells may develop along the way to create a series of two or more shorter dives instead of one long dive", Veni said, adding that "lower water levels means the force of the water is less".
Teams of Thai and worldwide divers have since supplied them 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%.
"We did an assessment and we think it is quite risky at this point to bring them out".
In letters home, written on water-stained paper and posted Saturday on Facebook, the boys and their coach sought to reassure their families that they were in good hands and in good spirits. The 13 are having health evaluations and rehabilitation, and are being taught diving skills.
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.
Rescuers have been pumping water around the clock in an effort to drain the deluged cave enough so that the boys can safely exit the cave.