(Fox News) -- At least four boys have been rescued from a cave in Thailand on Sunday, hours after 18 divers began their operation to free the 12 boys and their soccer coach, who have been trapped in the flooded cavern more than two weeks, Thai Navy SEAL said. 

The first two boys rescued from the cave were taken to the field hospital near the cave, Tossathep Boonthong, chief of Chiang Rai's health department, told Reuters

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“We are giving them a physical examination,"  Boonthong said.

At least two ambulances and helicopters were later seen leaving the scene. Officials had said earlier that helicopters were on standby to take anyone rescued from the cave to a hospital.

A group of 18 divers — 13 foreign and five Thai — began the operation at 10 a.m. Sunday local time to bring the boys, ages 11-16, and their 25-year-old coach out of the Tham Luang Nang Non cave in Chiang Rai province after the area saw mild weather and falling water levels in the cave. Chiang Rai acting Gov. Narongsak Osatanakorn said two divers will accompany each boy as they are led out of the cave.

“Today is a D-Day for our operation,” Narongsak said.

"I confirm that we are at war with water and time from the first day up to today," Narongsak added Saturday. "Finding the boys doesn't mean we've finished our mission. It is only a small battle we've won, but the war has not ended. The war ends when we win all three battles — the battles to search, rescue and send them home."

President Trump tweeted the U.S. is "working very closely" with the Thai government during the rescue mission. 

"The U.S. is working very closely with the Government of Thailand to help get all of the children out of the cave and to safety. Very brave and talented people!" Trump tweeted.

The start of the rescue mission brought in a sense of optimism over people in Thailand. Rescuers part of the Thai navy SEAL partaking in the operation posted on Facebook showing hands locked with a caption vowing to bring the boys home.

"We, the Thai team and the international team, will bring the Wild Boars home,” referencing the boys’ soccer team name.

Though four boys have been rescued as of Sunday night, the mission to extract the rest could take two to four days depending on the conditions in the cave. The acting Chiang Rai governor had said rescuers couldn’t wait for perfect weather conditions because of several factors. Incoming monsoon rains could once again flood the cave and deplete oxygen levels. 

The boys and their coach became stranded when they went exploring in the cave after a practice game June 23. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.

The Thai Navy said Saturday three navy SEALs, one a doctor, were with the boys and coach. The 13 were having health evaluations and rehabilitation, and were being taught diving skills. Food, electrolyte drinks, drinking water, medicine and oxygen canisters have been delivered to them.

In letters written by the boys released Saturday, the boys appeared calm and reassured their families they were “fine.”

"Don't be worried, I miss everyone. Grandpa, Uncle, Mom, Dad and siblings, I love you all. I'm happy being here inside, the navy SEALs have taken good care. Love you all," wrote one of the boys named Mick.

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