Football stars urged the Thai team trapped in a cave to "stay strong" as messages of support and concern poured in for the 12 boys and their coach.
Players from Brazil legend Ronaldo to England's John Stones said they were closely following attempts to rescue the team, whose ordeal coincides with the World Cup in Russia.
Many fans on social media said the boys deserved the World Cup trophy for their bravery after spending two weeks trapped in darkness underground.
"It is terrible news and the world of football hopes that someone can find a way to take these kids out of there," said Ronaldo at a FIFA media event, according to CNN.
The Wild Boars team, aged 11-16, became trapped by rising floodwaters after setting off to explore the cave with their 25-year-old coach after training on June 23.
Liverpool manager Jurgen Klopp told the team, "You'll never walk alone", quoting the motto of the English side which has a large following in Thailand.
"Stay strong and know we are with you," Klopp said in a video message sent to CNN.
"We are following all the news and hoping every second that you see daylight again. We are all very optimistic that it will happen, hopefully in minutes, hours or the next few days."
Stones said he had been discussing the cave rescue with his England teammates ahead of their World Cup quarter-final against Sweden.
"I've been speaking about it with a few of the boys," the defender said, according to British media. "It's so sad to see where they are and we hope they get out safe and sound."
World body FIFA told CNN it was in close contact with the Football Association of Thailand, while Croatia's football authority said it was "awed" by the team's calm under pressure.
"We are awed by the bravery and strength that these young boys and their coach have shown amidst such frightening circumstances," the Croatian Football Federation said on its website.
"These are situations that are bigger than sports, but their sporting competitiveness will help them cope with the current challenge."