"All conditions are still as good as they were yesterday", he told a news conference. Officials feared that an incoming storm could send water flooding back into the cave and make an escape even more hard for the boys, ages 11 to 16, and their 25-year-old coach. Heavy rains struck the area and parts of their path back to the cave entrance became swollen with floodwaters.
The second phase of the rescue effort began on Monday, hours after the first four were pulled to safety late Sunday.
They are being kept separate from their families at present because of infection fears, authorities said.
The remaining nine boys are still at the location where divers had found them about 500 metres from the so-called Pattaya Beach.
The mission was paused overnight on Sunday for air tanks to be replaced. Authorities have said extracting the entire team from the cave could take up to four days, but Sunday's success raised hopes that it could be done faster.
According to the Associated Press, a Thai official revealed that the healthiest boys were taken out of the cave first.
Prime Minister Prayuth Chan-ocha visited the cave to inspect the operation, with navy rescuers giving him a rousing cheer.
"We'll start preparing for the next rescue operation, and we believe we'll do better than today and it will be highly successful", the rescue mission chief said. On Sunday, the first four boys were rescued and were taken to a hospital in Chiang Rai for evaluation. Thai Navy SEALs and worldwide cave diving experts brought out four boys Sunday, and the operation is continuing today. The device could be used to rescue one boy at a time as it has space to fit in one person with his arms crossed, he said.
The death Friday of a former Thai navy SEAL, Saman Gunan, underscored the risks. Divers held the first four boys close to bring them out, and each had to wear an oxygen mask to enable normal breathing, authorities said. Cave rescue experts have said they consider an underwater escape to be a last resort, especially with people untrained in diving.
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A total of eight boys have now been rescued.
The oxygen ports and nose cone fitted to the device would prevent damages when it comes in contact with rocks, he said.
Thai authorities say they have resumed operations to rescue members of a boys' soccer team trapped in a flooded cave after successfully getting four of the boys out on Sunday.
Rescuers are moving ahead quickly because they are afraid monsoon rains will completely flood the caves.
The ordeal has riveted Thailand and captured the world's attention.
CNN reported that the rescue operation had resumed.
Interior Minister Anupong Paochinda also said the boys were in good health. On Sunday, he shared photos and video on Twitter showing a "tiny, kid-size submarine" that could potentially help with the rescue effort.
While swimming, each boy is helped by two divers as they follow a rope called a dive line, which helps lead the group out of the cave. However, it is unclear if the device is part of any current rescue plans.
Narongsak said he was not sure if the remaining five people will be extracted in one or more operations. "Very courageous and talented people!"