"We tried to pump the water (out of the chamber) but the water keeps rising. That means the water that comes in with the rain is still much more than what we can pump out," said Chiang Rai Gov. Narongsak Osatanakorn. "So we need to pump the water faster."
He said the navy SEAL teams leading the search have been bolstered.
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Narongsak also said teams continued to look for another way in to the blocked portions of the cave by searching for any shafts that might be accessible on the surface of the mountain, under which the cave sits. Two fissures were found Tuesday but proved to be dead ends. The rescuers were exploring another one found Wednesday morning.
Despite setbacks, officials remained publicly optimistic about the prospects for a successful rescue.
"We still have hope. All agencies are trying their best. We have a challenge from the water level that keeps rising," Thai army chief Gen. Chalermchai Sittisart told reporters at the site. "We are adding more pumps to lower the level down so that the SEAL team could operate better."
He added that all agencies were working hard, "rotating 24 hours so that we can have a success in this operation."
Interior Minister Anupong Paojinda told reporters earlier that the divers could 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.
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.
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.
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.