(CBSNEWS/AP) – Rescuers in northern Thailand on Sunday extracted at least four members of a youth soccer team from the cave where they had been trapped for more than two weeks, part of an ongoing operation to rescue the 12 boys and their coach, officials said.

The boys, ages 11-16, and their 25-year-old coach became stranded when they went exploring in the cave after a practice game. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.

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Narongsak Osottanakorn, who is in charge of the rescue mission, said Saturday conditions were currently “perfect,” BBC News reports.

“Now and in the next three or four days, the conditions are perfect [for evacuation] in terms of the water, the weather and the boys’ health,” said Narongsak, governor of the Chiang Rai province, where the caves are.

Engineers sent by Elon Musk remain on standby

The California tunnel company run by Elon Musk is continuing to maintain a presence at the Thai cave. Sam Teller, spokesman for Boring Co., said Sunday that the company has four engineers who are “offering support in any way the government deems useful.”

Musk tweeted early Saturday that he was working with a team from his SpaceX rocket company to build a “tiny, kid-size submarine” to transport the children. But Saturday night, he tweeted that the cave was now closed for the rescue by divers. “Will continue testing in LA in case needed later or somewhere else in the future,” he wrote.

Musk says the sub would be light enough to be carried by two divers and small enough to get through narrow cave gaps.

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Rescue operation going “better than expected,” next phase to begin in 10-20 hours

A Thai official says the next phase of the operation to rescue the rest of the trapped boys will begin in 10 to 20 hours as rescuers replenish their oxygen tanks.

Speaking to reporters at a briefing near the cave on Sunday night, Chiang Rai provincial acting Gov. Narongsak Osatanakorn said the healthiest boys were taken out first, with the first two emerging at 5:40 p.m. and 5:50 p.m. local time, or 6:40 a.m. ET. Two others were brought out about two hours later.

Narongsak said the operation was going “better than expected.” The four freed boys have been transported to the hospital, where they will be reunited with their families. Their condition was not known.

Trump says U.S. “working very closely” with Thai government on rescue

In his first tweet of the day, President Trump says the U.S. is working with Thai officials in the rescue operation:

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(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)