Officials late on Sunday revised down the number of boys rescued from the Tham Luang cave to four.

After initial reports of six boys reaching freedom after a difficult journey through flooded underground rivers, Thai officials said four boys were being treated in hospital.

They were said to be the strongest children.

Another phase of rescues is expected within 10 to 12 hours.

The first two boys walked on their own to a field hospital set up outside the cave on Sunday. T

Khaosod reported at 6pm (9pm AEST) that the first boy walked out at 5.40pm and the second at 5.50pm. It reported that they were being assessed by a medical team.

The news was confirmed by Chiang Rai health department chief Tossathep Boonthong, who told Reuters: “Two kids are out. They are currently at the field hospital near the cave.

“We are giving them a physical examination. They have not been moved to Chiang Rai hospital yet.”

The good news came well ahead of schedule, which predicted the boys were not expected to emerge until midnight, Australian time.

It came after the weather had started to turn.

The heavy rain that many had tipped started to fall mid-afternoon near the Tham Luang cave complex, where the rescue operation for 12 boys and their soccer coach began on Sunday morning.

Having been evacuated with all other journalists and non-essential personnel from the cave around noon, Australian time, The Australian journalist Amanda Hodge tweeted at 6.22pm that the downpour had begun as rescuers continue to wage a war between “water and time”.

An earlier shower had brought less rain than expected, and did not interfere with the electricity within the caves.

Ambulances arrived at the site in preparation for the extraction mission as dark monsoon clouds loomed.

A total of 18 divers entered the cave in Thailand’s northern province of Chiang Rai, where 12 schoolboys and their soccer coach have been trapped for two weeks.

Thirteen of the divers are foreign, including eight Australians.

The torrential rain everyone has feared has arrived, drenching the media centre just kilometres from #Thamluangcave where a rescue mission is currently underway. pic.twitter.com/CGH8iA89vr

— amanda hodge (@hodgeamanda) July 8, 2018

Retrieving the boys from the muddy bank on which they took refuge requires an 11-hour round trip through four kilometres of winding, submerged pathways, tunnels and fast-flowing, freezing water.

Some of the submerged tunnels are less than a metre high.

Two divers will gradually escort each boy out, one at a time.

Officials had said the earliest divers will emerge is 9pm on Sunday (midnight AEST), but a Thai official says the rescue of all the boys could take up to four days.

Preparations by the Thai Navy have been in full swing for more than 24 hours. Photo: Thai Navy/Facebook

Thirteen medical teams stand ready outside the cave, each with its own helicopter and ambulance, to assist each boy and their coach.

Medical staff say their first assessments will focus on the boys’ breathing, signs of hypothermia and an airborne lung infection known as ‘cave disease’.

Major General Pramote Imwattana of the Army Medical Department said the boys will then be driven by ambulances to makeshift helipads and airlifted to the Chang Rai hospital 70 kilometres away.

If efforts over the next few days fail, authorities have not ruled out laying an air line and leaving the boys inside the cave for months until monsoon rains clear.

Journalists and non-essential personnel are ordered to leave the cave site and surrounding roads as the rescue operation begins on Sunday. Photo: Getty Images

The Australian divers, in support of the Royal Thai navy, have been deployed under the Australian Medical Assistance Teams humanitarian program.

This takes the total number of Australians from DFAT, Defence and AFP involved in the rescue operation to 19.

-with AAP