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Experts have warned that it could take up to four days to complete the rescue of eight boys and their football coach from inside a northern Thailand cave.

Authorities temporarily stopped rescue efforts on Monday (local time) to replenish air tanks along the cave's treacherous exit route.

Expert divers on Sunday managed to get four of the 12 boys to safety who were quickly transported to a hospital in the town of Chiang Rai, the provincial capital.

The names of the rescued boys were not released.

Eighteen divers went into the Tham Luang Nang Non complex in northern Thailand at about 4am UK time today to begin to bring out the 12 boys and their coach who have been trapped for 15 days.

The Thai official leading the rescue operation said it has gone "better than expected" and that the healthiest boys have been taken out first.

The next phase of the operation will start in 10 to 20 hours, according to authorities.

Chief of operations governor Narongsak Osatanakorn said 13 foreign and five Thai divers are taking part in the operation.

The British Cave Rescue Council (BCRC) confirmed that seven divers from the UK with "expertise in cave diving" are assisting.

The Thai Navy SEALs posted on Facebook shortly before 8pm that four of the Wild Boars - the name of the young boys' football team - had been rescued.

Ambulances were seen driving to a nearby helipad, and a helicopter was seen taking off after the boys were extracted.

Officials had said earlier that helicopters were on standby to take anyone rescued from the cave to a hospital.

Two elite British divers, Rick Stanton and John Volanthen, were the first rescuers to reach the group on Monday night and are believed to be part of the team.

A BCRC spokesman said: "The UK divers are part of the core team, so they will be actively involved and that will include escorting each child out through the flooded passage.

"The operation is being supervised by the Thai authorities.

"They have had to make a quick decision because they are really concerned about the water level rising."

Mild weather and lowering water levels led officials to declare “today is D-Day” as the best opportunity to launch the rescue mission ahead of forecast heavy rain.

Expert divers entered the sprawling complex for the complicated and dangerous mission this morning.

Chiang Rai province acting governor Narongsak Osatanakorn, who is heading the operation, said 13 foreign and five Thai divers were taking part in the rescue and two divers will accompany each boy as they're gradually extracted.

"Divers will work with doctors in the cave to examine each the kids' health to determine who should get to come out first," Narongsak said.

The only way to bring them out of Tham Luang Nang Non in Chiang Rai province is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.

A former Thai navy SEAL passed out and died making the dive Friday.

Officials hope all of the boys and their coach will be free over the next two to four days but that is dependent on the weather not worsening.

The path out is considered especially complicated because of twists and turns in narrow flooded passages.

But the governor supervising the mission said earlier that mild weather and falling water levels over the last few days had created optimal conditions for an underwater evacuation that will not last if it rains again.

Before announcing that the rescue was under way, authorities ordered the throngs of media that have gathered at the cave from around the world to leave.

The boys, aged 11 to 16, and their 25-year-old coach became trapped in the cave after going in on June 23, following a practice match.

Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.

Authorities had said that incoming monsoon rains that could send water levels in the cave rising, coupled with falling oxygen levels in the enclosed space, added to the urgency of getting those trapped out.

Earlier efforts to pump out water from the cave have been set back every time there has been a heavy downpour.

The ordeal has riveted Thailand and made global headlines, and the search and rescue operation has involved international experts and rescuers.

President Donald Trump said in a tweet on Sunday: "The U.S. is working very closely with the Government of Thailand to help get all of the children out of the cave and to safety. Very brave and talented people!"

An update Saturday from the Thai navy said three navy SEALs were with the boys and their coach, one a doctor. The 13 were having health evaluations and rehabilitation, and were being taught diving skills. Food, electrolyte drinks, drinking water, medicine and oxygen canisters have been delivered to them.