ABC News' Matt Gutman reports on the ongoing effort in Thailand to rescue the trapped soccer players and their coach. (1:06)

Rescuers in Thailand retrieved four of the 12 boys who have been trapped with their soccer coach in a cave for two weeks in the first phase of an operation that could take days.

Chiang Rai provincial acting Gov. Narongsak Osatanakorn said the four healthiest boys were taken from the cave first. The rescued children were taken to the hospital, and officials said early Monday that they are strong and safe but need to undergo detailed medical checks.

In comments released by the government, Interior Minister Anupong Paojinda said the same divers who took part in Sunday's rescue will also conduct the next operation as they know the cave conditions and what to do. He said officials were meeting Monday morning about conducting the next stage of the operation.

Anupong said divers need to place more air canisters along the underwater route to where the boys and their coach have been trapped since June 23. He said that process can take several hours.

The boys and their coach had been stranded in Tham Luang Nang Non when they went exploring in the cave after a scrimmage. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.

Narongsak, who is heading the operation, said earlier Sunday that 13 foreign and five Thai divers were taking part in the rescue, and two divers were to accompany each of the boys, all of whom have been learning to dive only since Monday, when the first searchers found them.

An ambulance carries one of the boys rescued from Tham Luang Nang Non cave in Thailand. Thai navy SEALs say four of the 12 soccer team members are out of the cave. Linh Pham/Getty Images

The entire operation to get all 13 out of the cave could last two to four days, depending on weather and water conditions, army Maj. Gen. Chalongchai Chaiyakam said.

The only way to bring everyone out of the cave is by navigating dark and tight passageways filled with muddy water, strong currents and oxygen-depleted air. A former Thai navy SEAL passed out while making the dive Friday and died.

Mild weather and falling water levels over the past few days created optimal conditions for an underwater evacuation, but that won't last if it rains again, Narongsak said.

The next phase of the operation would start Monday after rescue teams replenish the supply of oxygen tanks along the route to ensure the safety of the journey, which takes several hours.

The Associated Press contributed to this report.