Four of 12 Thai schoolboys were rescued from a flooded cave on Sunday as divers launched a daring and dangerous mission to free the children and their soccer coach who have been trapped underground for more than two weeks, officials said.

Thirteen foreign divers and five members of Thailand's elite navy SEAL unit guided the boys to safety through narrow, submerged passageways that claimed the life of a former Thai navy diver on Friday.

As night fell, the operation to rescue the remaining eight boys - some as young as 11 and weak swimmers - and their coach was called off until Monday morning.

"Today we managed to rescue and send back four children to Chiang Rai Prachanukrua Hospital safely," the head of the rescue operation, Narongsak Osottanakorn, said at a news conference.

The rescue teams needed about 10 hours to prepare for their next operation, involving about 90 divers in total, 50 of them from foreign countries, he said.

A helicopter flew the four boys to the nearby city of Chiang Rai, where they were taken by ambulance to hospital.

Their ordeal has drawn huge media attention in Thailand and abroad, and getting the boys out safely could be a boost for Thailand's junta ahead of a general election next year.

"Today is D-Day," Narongsak had earlier told reporters.

Bursts of heavy monsoon rain soaked the Tham Luang Cave area in northern Chiang Rai province on Sunday and storms were expected in the coming weeks, increasing the risks in what has been called a "war with water and time" to save the team.

The boys, aged between 11 and 16, went missing with their 25-year-old coach after soccer practice on June 23, setting out on an adventure to explore the cave complex near the border with Myanmar and celebrate a boy's birthday.

The rescue teams had rehearsed the plan for several days, Narongsak said, and had managed to drain the water level in the cave considerably, but needed to move fast.

"If we wait and the rain comes in the next few days we will be tired again from pumping and our readiness would drop. If that's the case, then we have to reassess the situation," he said.

An Australian doctor who is part of the rescue mission checked the health of the boys on Saturday night and gave the all-clear for the operation to proceed.

The boys were discovered by British divers Richard Stanton and John Volanthen on Monday.

Of the 13-strong foreign dive team - mainly from Europe - three were escorting the children, while the remainder were positioned along the dangerous first kilometre stretch, where the boys will have to navigate through submerged passageways in some places no more than two feet (0.6 metre) wide.

MEDICAL TEAMS ANXIOUS

Thirteen medical teams were stationed outside the cave - each with its own helicopter and ambulance - one for each of 12 boys and their coach.

A source at the Chiang Rai hospital said that five emergency response doctors were awaiting the boys and a further 30 doctors were on stand-by.

"The teams here are happy the boys are being rescued but also anxious about the severity of the boys' conditions. We're under a lot of pressure," she said, declining to be named because she was not allowed to speak to the media.

The area outside the hospital was cordoned off with police patrolling the area. Down the street, a loud speaker told vendors to "keep off the road" and to "not obstruct the transfer mission".

Prime Minister Prayuth Chan-ocha, leader of the military junta that seized power in 2014, planned to visit the cave site on Monday, a government spokesman said.

His visit with relatives and rescue officials last week was criticised by some Thais as opportunistic as his government faced pro-democracy protests in the capital Bangkok in recent months.

US President Donald Trump said the United States was working "very closely" with the Thai government to free the boys.

"Very brave and talented people!" he tweeted.