Thai cave rescuers at 'war' against water in race to evacuate boys' soccer team Conditions have improved, but that could all change if rain returns.

Decreasing water levels and relatively dry weather over the past few days have created more favorable conditions for evacuating the boys' soccer team trapped inside a partly-flooded cave in northern Thailand, according to the local official in charge of the operation.

But as that could all change if rain returns, rescuers are "racing against the clock," the official, Narongsak Osatanakorn, told reporters at a news conference Saturday.

"We are still at the state of war against the water. All the plans must not have any holes in them," he said, noting that "hundreds of people have vetted this" and "there will always be margins for error."

Narongsak said Friday that one plan under consideration is to bring the 12 boys and their soccer coach out of the Tham Luang Nang Non cave in mountainous Chiang Rai province the same way they initially entered during a hike two weeks ago. The group has been learning how to breathe underwater using dive equipment, while rescue crews pump floodwater out from the cave and drill into the rock formation around it, looking for other ways to safely evacuate the trapped group.

"We are carrying the weight of this mountain on our shoulders," Narongsak told reporters Saturday. "Eventually, we'll choose a plan that we'll execute and have to complete."

Seasonal monsoon rains forecast to hit the region this weekend and throughout next week have yet to occur, and efforts on the ground to remove floodwater and divert water flows have been "very successful," Narongsak said. Rescuers can now walk, rather than swim or dive, from the cave's main entrance toan inner chamber serving as a command center.

"We are very happy with the water situation here," he said. "The perfect situation would be to have zero water, which is impossible. The water level would be zero during December or January, so this situation is absolutely impossible. The next best situation would be if the water level is as low as possible to move the kids."

Thailand's wet monsoon season typically lasts from May to October, and the Tham Luang Nang Non cave acts as a natural underground drainage system for the surrounding region.

Rescue crews have also been scouring the region's rugged jungle-covered terrain for any natural shafts that connect to the cave network below. They have drilled more than 100 holes and dug into 18 of them, which showed potential. They drilled 400 meters down one of them, but it didn't line up to where the soccer team is located, according to Narongsak.

Oxygen levels running low

Despite the progress, Narongsak said rescuers are struggling to maintain safe oxygen levels inside the cave.

The inner chamber where rescuers have set up a "forward operating base" now has low oxygen levels due to the amount of people inside. They have tried pumping "pure air" through a tube into the chamber, but have also had to pull back nonessential personnel in an effort to preserve oxygen, according to Narongsak.

"We have experienced a lot of people fainting inside," he said. "We want to keep the headcount minimal, but we'll always have four people with the kids and we'll work hard to bring as many oxygen tanks into that area as we can."

The Associated Press reported that Narongsak said rescuers were waiting for two large groups of volunteer foreign divers to arrive later Saturday and Sunday, after which the evacuation effort could begin quickly when the conditions are right.

However, Narongsak made no mention during Saturday's news conference about awaiting additional teams of divers.

'Buddy dive' detailed in internal U.S. government report

The Royal Thai Navy -- supported by divers from the United Kingdom, the United States and other nations -- has briefed Thai military leadership, interior ministry officials and the provincial governor on a proposed operation to evacuate the boys and their coach from the cave alongside experienced divers, in what is being called a "buddy dive," according to an internal U.S. government report obtained by ABC News on Friday. The document said Thailand's prime would be briefed on the proposal Saturday morning local time.

Despite any risks in such an operation, an accelerated timeline would take advantage of the cooperating weather and the children's still having high oxygen levels within the cave complex and being in relatively good health, according to the document.

The document indicated that the proposed plan, if approved, could launch as soon as this weekend. But a definitive timeline was unknown.

The first phase of the proposed operation involves an ongoing process of staging equipment and clearing obstacles in the cave. The second phase calls for dangerous and risky “buddy diving” of the soccer team, in which each of the boys and the coach would be accompanied by an experienced diver to outside the cave network.

U.S. dive and medical personnel would support the proposed operation by staging equipment and setting up triage stations, but would not go beyond the third chamber inside the cave network, according to the document.

The death of a former member of the Royal Thai Navy volunteering for the rescue effort has hindered some progress. Saman Gunan lost consciousness underwater during an overnight operation delivering extra air tanks along a treacherous route divers take to get to the trapped soccer team. He could not be revived and was confirmed dead early Friday morning.

Gunan, 38, formerly served in the Royal Thai Navy’s Underwater Demolition Assault Unit, colloquially known as the Thai Navy SEALs. His death marked the first fatality in the operation to rescue the group and underscored the dangers of navigating through the cave underwater, even for those who have experience.

"We will learn from our mistakes and try not to repeat them again," Narongsak told reporters Saturday. "I will have to use the word 'try' here because I cannot make a commitment that something like this won't happen again. We can only use the words 'try' and 'do our best.'"

A plan to install an oxygen cable in the chamber also had to be abandoned due to difficulties in routing the cable through the cave’s labyrinth of caverns and narrow passageways, according to the document.

Pumping has not been able to significantly lower water levels deep within the cave network, according to the document obtained by ABC News. Rescue crews have determined that pumping water from the main entrance of the cave is becoming less effective, and placing pumps further inside the cave does not seem possible, according to the report.

Medical assessment indicate the trapped children have good oxygen levels, with all of them having oxygen saturation levels of 95 percent and above. The oxygen level within the cave was most recently measured at 17.5 percent. Though higher than the previous reading, it is still significantly below the normal range of 20 to 21.5 percent, according to the document.

'Like finding a needle in a haystack'

The boys, ages 11 to 16, and their 25-year-old soccer coach have been trapped inside Tham Luang Nang Non, Thailand’s longest cave, since June 23. It’s believed the coach often took the Wild Boar teammates to the cave in Khun Nam Nang Non Forest Park for a fun excursion after practice.

But as the group ventured deeper into the cave that Saturday afternoon, the sky opened up and it began to rain. The downpour sent floodwater rushing into the mouth of the cave and cut off their exit route. The group forged ahead until finding a dry, raised slope where they have remained stranded.

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. Persistent rain initially impeded efforts to locate the group. But on July 2, two British divers found all 13 alive in an area about three miles from the cave’s main entrance.

A team of Royal Thai Navy members together with a doctor and a nurse have been staying with the group, giving them high-powered protein drinks and medical assessments, while officials work on a plan to get them out as safely and quickly as possible.

"The search was like finding a needle in a haystack, but the rescue is proving much harder because the conditions we face are not normal," Narongsak told reporters Saturday. "Nowhere in the whole world have people ever faced conditions like this before."

ABC News' Brandon Baur, Joohee Cho, Matt Gutman, Aicha El Hammar, James Longman, Luis Martinez, Matt McGarry, Elizabeth McLaughlin, Scott Shulman and Robert Zepeda contributed to this report.