CHIANG RAI, Thailand (Reuters) - Four of 12 Thai schoolboys were rescued from a flooded cave on Sunday in a daring and dangerous operation to save the children and their soccer coach who have been trapped underground for more than two weeks.

The operation to rescue the remaining eight boys - some as young as 11 and weak swimmers - and the coach was called off at nightfall until Monday to give the divers time to replenish oxygen supplies and ensure all preparations were complete.

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.

“Today was the best day, the best situation in terms of the weather, the health of the boys, our water management for our rescue effort,” the head of the rescue operation, Narongsak Osottanakorn, told a news conference.

“Today we managed to rescue and send back four children to Chiang Rai Prachanukrua Hospital safely.”

The rescuers needed at least 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 the 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 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 cave water level considerably, but needed to move fast.

“CONDITIONS MUST BE STABLE”

Slideshow ( 21 images )

“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.

“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.

“...I can’t give you an exact number but it should be more than 10 hours but not exceeding 20 hours. The conditions must be stable like today before we can continue the operation.”

Slideshow ( 21 images )

An Australian doctor 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 escorted the children, while the remainder were positioned along the dangerous first kilometer stretch, where the boys had to navigate through submerged passageways in some places no more than two feet (0.6 meter) wide.

The area outside the Chiang Rai hospital was cordoned off by police. Down the street, vendors were told by loud speaker 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 criticized by some Thais as opportunistic as his government faced pro-democracy protests in the capital Bangkok in recent months.

(GRAPHIC: Thailand cave rescue: Hope for the 13 - tmsnrt.rs/2KR2zRj)