* Rescuers undertake second stage of cave operation

* First four boys freed said to be in good health

* Six more ambulances waiting near cave mouth

* Boys must navigate narrow, submerged passageways

By Patpicha Tanakasempipat and Panu Wongcha-um

CHIANG RAI, Thailand, July 9 (Reuters) - Rescue workers in Thailand carried a person on a stretcher from a cave into an ambulance on Monday, a Reuters witness said, apparently the fifth person to be rescued from a group of 12 boys and their soccer coach trapped for more than two weeks.

There was no immediate confirmation from authorities that the person on the stretcher was one of the boys trapped in the flooded cave complex but media reported it was.

The first four boys, from the group of 13, were brought out on Sunday and were in good condition in hospital, officials said.

The dangerous bid to rescue the boys - aged between 11 and 16 - got going again hours earlier on Monday after a break to replenish oxygen supplies and make other preparations deep inside the cave complex in northern Thailand's Chiang Rai province.

Authorities have said the mission could take three or four days to complete. It is a race against the clock with heavy rain expected in coming days, which would again dangerously flood the tunnels.

The chief of the rescue mission, Narongsak Osottanakorn, earlier told a news conference a team had gone into the cave to rescue the remaining boys at 11 a.m. (0400 GMT), and he expected good news soon.

The Reuters witness near the cave saw medical personnel carrying the person, wrapped in green sheets and lying flat, into an ambulance close to the mouth of the cave.

Six more ambulances were waiting nearby.

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

Narongsak said that the "same multinational team" that went into the cave on Sunday to retrieve the first four boys was deployed on Monday.

He did not say how many boys the team hoped to bring out on Monday.

The "Wild Boars" soccer team 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.

British divers found the 13, huddled on a muddy bank in a partly flooded chamber several kilometres inside the complex, on Monday last week.

Then the problem became how to get them out as more rainy season showers loomed.

The rescue began on Sunday. Divers held the first four boys close to bring them out, and each had to wear an oxygen mask to enable normal breathing, authorities said.

Interior Minister Anupong Paochinda told reporters the rescued boys were in good health in hospital but did not give details.

INFECTION FEARS

Authorities have not confirmed the identity of the first four boys rescued. Some of the boys' parents told Reuters they had not been told who had been rescued and that they were not allowed to visit the hospital.

Narongsak said the first four rescued boys had not been identified out of respect for the families whose sons were still trapped, adding that the boys were being kept away from their parents due to fear of infection.

"The four children are well at Chiang Rai Prachanukroh hospital. But they still need to be kept away from their parents and others due to fear about infection," he said.

Medical teams previously said concerns included hypothermia and an airborne lung infection known as "cave disease", which is caused by bat and bird droppings.

Somboon Sompiangjai, 38, the father of one of the trapped boys, said parents were told by rescuers ahead of Sunday's operation the "strongest children" would be brought out first.

"We have not been told which child has been brought out ... We can't visit our boys in hospital because they need to be monitored for 48 hours," Somboon told Reuters.

"I'm hoping for good news today," he said.

WORLD CUP FINAL

At least nine ambulances loaded with stretchers and blue oxygen tanks waited by the mouth of the cave on Monday. Soldiers, medics, engineers and volunteers in yellow shirts milled around but the mood was relaxed.

"I feel very happy, everybody is happy," said Hnin Jaiwong, the mother of one of the trapped boys, 13-year-old Sompong Jaiwong.

"I don't know if he is out, they didn't tell us," she said as she rested in a hut close to the mouth of the cave.

The cave complex is off-limits during the rainy season, which usually runs from May to October, when downpours can quickly flood it.

Relatives said the boys had been inside the labyrinthine complex during the dry season.

The fate of the boys and their coach has gripped Thailand and drawn international media attention.

The president of soccer's governing body, FIFA, has invited the boys to the World Cup final in Moscow on Sunday if they make it out in time.

(Reporting by Panu Wongcha-um, Patpicha Tanakasempipat, John Geddie and James Pomfret in CHIANG RAI Additional reporting by Amy Sawitta Lefevre, Panarat Thepgumpanat, Pracha Hariraksapitak, and Aukkarapon Niyomyat in BANGKOK Writing by John Geddie Editing by Amy Sawitta Lefevre, Robert Birsel)

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