The entire operation to get all 13 out of the cave could last two to four days, depending on weather and water conditions, army Maj.
Four boys were rescued on Monday evening following the four who were bought safely out of the cave on Sunday.
He had said fresh air tanks needed to be laid along the underwater route.
The risky operation to rescue the 12 boys and their coach began early Sunday morning and the entire operation involves 90 expert divers from around the world.
The next day (7th July), it was decided by the rescue operation chief Narongsak Osottanakorn that it was not yet safe for the boys to dive out of the cave.
"Imagine marathon runners. It's like when they reach the finish line exhausted", said the source.More news: Trump to Unveil Court Pick Monday
Interior Minister Anupong Paojinda had said early Monday that the same group of expert divers who took part in Sunday's rescue would return to extricate the others because they know the cave conditions and what to do.
"We can only carry on the operation once we are ready and this will be done soon, because the air tank and other systems have to be reinstalled", he told reporters later.
The boys and their coach set out to explore the vast Tham Luang cave complex after soccer practice on June 23, and got trapped when a rainy season downpour flooded the tunnels. The Associated Press reports that four ambulances were seen leaving the area near the cave entrance on Monday, with patients taken to waiting helicopters.
Just a few kilometres from the lush forest that covers the mountainside of the Tham Luang cave, four boys dressed in their white school uniforms shoot hoops on an outdoor basketball court in Mae Sai.
Ivan Karadzic, a Danish dive instructor who lives in Thailand and who was involved in the rescue effort of the four people on Sunday as a support diver, said late that night that "it went surprisingly well, we were expecting bad things to happen, and they didn't". They were stranded by flash floods that caused water levels to rise, making it hard to get out.
The death Friday of a former Thai navy SEAL, Saman Gunan, underscored the risks.More news: Leckie named a 'breakout star' of the World Cup
Elon Musk's Space X rocket company is testing a "kid-sized submarine" that could be sent to help boys trapped in a flooded Thailand cave.
Thai government announces that two boys have emerged from the flooded cave in which they were trapped.
The group was found nine days later in a cramped chamber 2.5 miles inside the cave system by two British volunteer rescue divers.
The submarine may or may not be necessary: at the time of writing, rescuers have already managed to extract four boys from the cave.
Rescuers in Thailand on Monday began their second attempt to free the remaining boys and their coach still trapped inside a cave, but warned that heavy rains have soaked the area and they are now in a race with "water and time".
The team still faces a "war against water" and a short window to rescue the youngsters, as monsoon rain has continued to fall.More news: Phil Thompson urges Liverpool to sign Harry Maguire
The soccer team had been trapped in the underground cave network since June 23, with flash floods causing water levels to rise, making it hard to get out.