Flood waters seeped into a second chamber of the Tham Luang cave and forced some of the rescue divers to turn back, according to officials.
The children, aged between 11 and 16, went into the cave Saturday and were trapped when heavy rains clogged the main entrance.
Around 1,000 Navy SEAL divers, police, soldiers, border guards and officials have been mobilized for the around-the-clock rescue in a remote and mountainous part of Chiang Rai province near the Laos and Myanmar borders.
A team of American military personnel from the US Pacific Command, including pararescue and survival specialists, arrived at the site overnight to help rescue operations, according to embassy spokeswoman Jillian Bonnardeaux.
"Operators are trained in personnel recovery tactics and techniques and procedures," she told AFP.
"Essentially what they're looking at is assessing with the Thai authorities the potential courses of action and complementing the efforts underway."
Three British cave diving experts made a second attempt at entering the cave through chimneys Thursday after they tried to go in through the main entrance on the day before but were turned around by rushing flood waters.
At nearly 10 kilometers, Tham Luang cave is one of the longest in Thailand and has the reputation of being one of the toughest, even among experienced divers.
It frequently floods during Thailand's monsoon season between July and November, rendering some of its narrow passages difficult to pass through.
There are several air pockets within the complex and the kids are believed to be in a large chamber in the middle of the cave.
But fast-running and murky flood waters have prevented rescue divers from reaching them and heavy rains continue to seep into the cave through narrow openings on the sides and through the main entrance.