To get there, search and rescue teams including at least 90 divers have to pass through more than two-kilometers of narrow, flooded tunnels.
Two ambulances left the cave and drove to a nearby helipad, and a helicopter was seen taking off.
Authorities have said helicopters were ready to take cave evacuees to a hospital.
For the boys - some of whom can't swim - the most risky part of the journey out of the labyrinth cave system remains the first kilometer, in which they are required to pass through a flooded channel no wider than a person.
Now only five members of the team remain in the cavern complex, where they became trapped more than two weeks ago.
According to Thailand's interior minister, same divers who rescued 3 students and the coach on Sunday night will conduct the next operation as they are aware of the cave conditions.
During this process, rescuers need to hold the boys' oxygen tanks in front of them and swim pencil-like through submerged holes.Having completed this section, the boys are then handed over to separate, specialist rescue teams, who help assist them through the remainder of the cave, much of which they can wade through.
There are reports the divers re-entered the cave around 1pm local time Monday.
"The fact that it took so many hours underscores how precarious this whole mission is", she said. While there was no immediate word on their condition, Dr. Ashton said doctors will likely conduct a battery of tests on the boys to evaluate not only their physical condition but their mental condition as well.
Thousands of people including soldiers, navy and police have been mobilised for the operation to save 12 young footballers and their coach from the Moo Pa team - or "wild boar" in Thai. Apart from the Thai authorities, 19 Australian divers are a part of the risky rescue mission.
It is believed they have been able to make the last part of the journey on foot.
His updates came after U.S. and Thai officials confirmed Sunday that four boys had been rescued from a flooded cave network after spending 15 days trapped along with their soccer team and coach.
"Air tanks and systems have to be put in place again", he added.
The province is expected to experience flooding on Monday and Tuesday, further delaying the rescue effort. They said rescuing the eight remaining boys and their soccer coach could take up to four days. Thailand's Meteorological Department said there was a 60 percent chance of rain Monday with thunderstorms forecast throughout the week, the Associated Press reported.
On Sunday, Musk posted photos and video of the "kid-size submarine" his team was testing in LA.
If the tests in California are successful, the submarine will be placed on a 17-hour flight to Thailand.
Narongsak said the rescued boys had not been identified out of respect for the families whose sons were still trapped, adding that the boys were being kept away from their parents due to fear of infection.
It was 10am (4am United Kingdom time) when officials made a decision to begin what one expert said was "beyond a shadow of a doubt, the most complex rescue ever attempted in the caving world".
Authorities temporarily stopped their efforts Monday to replenish air tanks along the cave's treacherous exit route.
Sunday, rescuers successfully navigated the first batch of four boys through the perilous escape route. Workers were seen lining the roads around Chiang Rai airport, the closest regional airport to the site of the cave, with flags representing the Thai king.
Now, the rescue operation is now racing against the clock as heavy rains pound the area and the water is dangerously rising in the caves.
The entire group had been trapped for more than two weeks. The device is also outfitted with oxygen ports and a nose cone to protect it from impact with rocks, according to Musk's tweets.
But Saturday night, he tweeted that the cave was now closed for the rescue by divers.