More heavy rain is expected at the weekend, which is set to complicate the rescue effort.
The ordeal facing the boys was underscored by the death of a former Thai Navy SEAL in the cave on Friday morning. "We will not let our fallen brother die in vain".
"We are following all the news and hoping every second that you see daylight again", Klopp said.
However, Ivan Katadzic, a Danish diving instructor who has been ferrying oxygen tanks inside the cave, said after a dive on Friday evening he was "double positive" about the mission because the water level had dropped considerably.
The letter signed by Federation Internationale de Football Association president Gianni Infantino also said the organisation would like to invite the youngsters and their coach to the World Cup final in Russia, should they be rescued in time and are healthy enough to travel.
It has been suggested that they may need to learn how to scuba dive in order to escape, but none of the children can swim and cave diving is a notoriously unsafe sport, as emphasised by the death of one of their rescuers, Saman Gunan, a former navy diver, in the caves on Friday.
"A navy SEAL just passed away last night".
Rafael, 53, who is from Israel but has lived in Thailand for more than 30 years, said he had been inside the cave 25 years ago and found that it was more hard to navigate than other caves he had tackled.
"There will be rain and many things could go wrong. I want to say thanks for all the support and I want to apologise to the parents".
Thai rescue teams gather, June 25, 2018.
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Authorities have been racing to pump out water from the cave before more storms raise the water level again. They have been working on a five km (three miles) "oxygen pipeline" to prepare for the group's extraction.
After they didn't return from their hike, Thai officials launched a massive search and rescue operation involving more than 1,000 people, including specialists drafted from various nations such as Australia, China, Japan, the United Kingdom and the United States.
The 12 boys and their coach have been trapped in the cave since June 23 after water levels rose and forced them to take refuge on an internal plateau inside.
A family member looks at a photo near Tham Luang cave complex, as members of under-16 soccer team and their coach have been found alive according to a local media's report in the northern province of Chiang Rai, Thailand, July 2, 2018.
Worldwide rescuers team prepare to enter the cave where a young soccer team and their coach trapped by flood waters.
The group can not be brought out of the cave immediately because they are not physically strong enough to do so and the conditions in the flooded cave remain unstable.
But getting them out won't be easy. A diver who spent time with the boys said earlier the youngsters wanted to know the results of the matches. Some of the boys can not swim. Narongsak Osatanakorn, the Thai official in immediate charge of the rescue operation, told reporters at a news conference late Friday that the plan was to bring the boys and their coach out of the cave the same way they initially entered two weeks ago.
Thailand's monsoon season runs from July to October and, while the past few days have been relatively dry, the long term forecast is rain for months.
But rescuers now have to figure out how to get them out, through several kilometres of dangerously flooded tunnels, or even to attempt to bring them out for now.
Authorities continue to pursue an alternative option, which is finding a shaft or drilling into the mountain in which the cave is located to find a sort of back door entrance.