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Rain continued to fall and water levels kept rising Wednesday inside a cave in northern Thailand, frustrating the search for 12 boys and their soccer coach who have been missing in the sprawling underground caverns for nearly four days.

At the entrance to Tham Luang Nang Non cave in Chiang Rai province, officials were bringing in large water hoses and more water pumps. Navy SEAL divers who have been working their way through the complex said water levels were rising in some places at a rate of 15 centimeters (6 inches) per hour early Wednesday.

Interior Minister Anupong Paojinda told reporters Tuesday that the elite divers leading the search were seriously handicapped by muddy water that has filled some chambers of the large cave to the ceilings. He said the divers can proceed only when enough water is pumped out so there is space between the water and the ceiling to make it safer to work. The divers will also soon start using special oxygen tanks that provide longer diving times, he said.

A SEAL diver said the water is so murky that even with lights they cannot see where they are going underwater, so they need to be able to lift their heads above the water.

Anupong said rescuers would be working night and day in the dark cave.

The boys aged 11-16 have been missing since their 25-year-old coach took them to the cave complex on Saturday after a practice match. Officials have previously said they believe they are still alive.

Somkuan Saokeaw, a volunteer rescue worker who had just left the cave area where a SEAL team was operating, said Wednesday morning that they had worked through the night in the farthest chamber they could access.

Divers have been seeking a way forward through the chambers of the cave complex, but have been forced to suspend their search several times. The authorities are also seeking alternative ways in, using helicopters and search parties on foot to find possible holes in the ceilings of other parts of the cave.

A small glimmer of hope was dashed Tuesday afternoon after explorers found two fissures in the rock on the mountain in which the cave is located. After evaluation, experts found neither could be used as a "chimney" to gain access to the cave.

The cave complex extends several kilometers (miles) and has wide chambers and narrow passageways with rocky outcrops and changes in elevation. Still, officials have said they are hopeful the boys found a safe space away from the floods.

Parents waited overnight in tents outside the cave entrance as rain poured. Medics sat in a tent nearby, and bicycles, backpacks and soccer cleats the boys left behind remained at the entrance.

Authorities have said footprints and handprints were found inside the cave complex, as well as other items thought to belong to the boys. They noted that tourists trapped there by past floods have been rescued after the waters receded.

The cave, cut into a mountainside near the border with Myanmar, can flood severely during the rainy season, which runs from June to October.

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Associated Press journalists Kaweewit Kaewjinda and Jason Corben in Bangkok contributed to this report.