A military helicopter flies over the Tham Luang cave complex in the northern province of Chiang Rai, Thailand, July 9, 2018. (Photo: REUTERS/Soe Zeya Tun)

The mission which started Sunday is a race against the clock with heavy rain expected this week which would again flood the tunnels with fast-flowing, rising water.

A crack team of foreign divers and Thai Navy SEALS guided four boys on Sunday and a further four on Monday through narrow, submerged channels from the muddy bank deep inside the Tham Luang cave where they had been stranded for more than two weeks.

"All four boys have arrived at hospital, all are safe," Narongsak Osottanakorn, head of the rescue mission, told reporters of those freed on Monday.

The operation on Monday went more smoothly than on Sunday and took two hours less as the practice became more refined.

But Osottanakorn added rescuers may need more than 20 hours, to replan and replenish oxygen supplies, to be ready to extract the remaining four boys and their coach.

Then it will depend on the weather. Heavy rain has struck the region intermittently over the last three days and further downpours could set back draining efforts at the cave.

Prime Minister Prayuth Chan-ocha visited the cave to inspect the operation.