Narongsak Osatanakorn, former governor of Chiang Rai province, who is heading up the rescue operation said the earliest any boy could be brought out is 9pm.
A Thai army medic sits in an ambulance on standby for the rescue operation in a restricted area near Tham Luang cave.
Thai divers would lead the mission and United States divers would preposition oxygen tanks, the U.S. official said.
Thai military officials have been briefed, the source said, and the country's Prime Minister Prayut Chan-o-cha was to be told about the plan Saturday morning.
At 10 a.m. local time, an global contingent of 13 specialist divers descended into the watery network of underground tunnels below the Mae Sai mountains, carrying with them the hopes of an entire nation.
He said water levels had dropped 30cm since Saturday and "we have reached peak readiness".
The letters provoked a surge of emotion from families, who first endured 9 long days before their children were found disheveled and emaciated but alive on Monday, July 2 - and now face an agonizing wait for a risky evacuation.
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"Once the heavy rains were going to hit, it was always going to be the deciding factor".
In their reply to the handwritten letter of Ekkapol Chanthawong, or Coach Ake, they said they appreciated him for taking care of their kids.
The "Wild Boars" team has been trapped in a cramped chamber several kilometres inside the Tham Luang cave in northern Thailand since 23 June when they went in after football practice and were hemmed in by monsoon floods.
CNN's Matt Rivers said the forecast for monsoonal rains meant there might be no alternative but to try and get the boys out now.
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Rescuers found them 10 days later and are now working to find a way to get them out.
"If, as we all hope, they are reunited with their families in the coming days and their health allows them to travel, Federation Internationale de Football Association would be delighted to invite them to attend the 2018 World Cup final in Moscow as our guests", Infantino wrote.
It takes even the most experienced divers up to five hours to swim through jagged, narrow channels from where the boys are to safety outside.
Gong Hui, a Chinese diver involved in the operation that has drawn some 130 Thai and worldwide divers, told Reuters on Saturday before the fresh rains that water levels in the cave had "receded a lot" after sustained pumping had removed millions of liters of water.
A RESCUE of the boys' soccer team trapped underground in a Thai cave may be imminent, with conditions favourable for an extraction.
Narongsak said that rescuers have assessed the boys' condition and also briefed their families, who are supportive of the rescue plan.
Footage of the rescue operation showed one of the boys wearing a red England away shirt, while another was seen in the blue of Chelsea. The 38-year-old ran out of air while underwater, an official said.
Officials have declined to comment but a news conference is scheduled to take place shortly.
Finnish volunteer diver Mikko Paasi, a long-term resident of Thailand, said Kunan's death had changed the mood on the ground and made real for rescuers just how unsafe the mission had become. We would like you to feel relieved.
"Our main obstacles include water and time therefore we have to do every way and keep looking for the best timing to launch our operation". "I had been anxious about my son, that he would be exhausted, he would be tired", he said.
One boy, Mick, wrote: "Don't be anxious, I miss everyone".
They entered the site under brooding clouds spitting steady rain.
The site around the entrance to the cave was cleared of media and other people earlier today, sparking specualtion a rescue attempt was likely to begin. That could take months, however, given that Thailand's rainy season typically lasts through October.