Thai soldiers bring hoses and additional water pumps to continue the search for 12 young soccer team members and their coach after going missing in Tham Luang Nang Non cave in northern Thailand. Picture: Sakchai Lalit/AP
Thai soldiers bring hoses and additional water pumps to continue the search for 12 young soccer team members and their coach after going missing in Tham Luang Nang Non cave in northern Thailand. Picture: Sakchai Lalit/AP
Rescue personnel search for alternate entrances to a cave where 12 boys of a soccer team and their coach went missing in Mae Sai, Chiang Rai, northern Thailand.  Picture: Thailand Department of National Parks and Wildlife via AP
Rescue personnel search for alternate entrances to a cave where 12 boys of a soccer team and their coach went missing in Mae Sai, Chiang Rai, northern Thailand. Picture: Thailand Department of National Parks and Wildlife via AP
Relatives of 12 young soccer team members and their coach pray for their rescue after going missing in a large cave in Mae Sai, Chiang Rai province, northern Thailand. Picture: Sakchai Lalit/AP
Relatives of 12 young soccer team members and their coach pray for their rescue after going missing in a large cave in Mae Sai, Chiang Rai province, northern Thailand. Picture: Sakchai Lalit/AP
A topographical map annotated with location details including entrance, water direction, dry ground, etc. of the Tham Luang Nang Non cave complex in which 12 young soccer team members and their coach went missing on Saturday. AP Photo
A topographical map annotated with location details including entrance, water direction, dry ground, etc. of the Tham Luang Nang Non cave complex in which 12 young soccer team members and their coach went missing on Saturday. AP Photo
Prayer offerings are displayed as rescue workers continue the search for the young soccer team and their coach that went missing in a cave in Mae Sai, northern Thailand. Picture: Tassanee Vejpongsa/AP
Prayer offerings are displayed as rescue workers continue the search for the young soccer team and their coach that went missing in a cave in Mae Sai, northern Thailand. Picture: Tassanee Vejpongsa/AP
One of the relatives of 12 young soccer team members and their coach prays for their rescue after going missing in a large cave in Mae Sai, northern Thailand. Picture: Sakchai Lalit/AP
One of the relatives of 12 young soccer team members and their coach prays for their rescue after going missing in a large cave in Mae Sai, northern Thailand. Picture: Sakchai Lalit/AP
Rescue personnel walk out of the entrance of a cave complex where it's believed that 12 soccer team members and their coach went missing in Mae Sai, northern Thailand. Picture: Sakchai Lalit/AP
Rescue personnel walk out of the entrance of a cave complex where it's believed that 12 soccer team members and their coach went missing in Mae Sai, northern Thailand. Picture: Sakchai Lalit/AP
Emergency rescue teams gather in the staging area as they continue the search for 12 young soccer team members and their coach after going missing in a large cave in Mae Sai. Picture: Sakchai Lalit/AP
Emergency rescue teams gather in the staging area as they continue the search for 12 young soccer team members and their coach after going missing in a large cave in Mae Sai. Picture: Sakchai Lalit/AP

Chiang Rang, Thailand - Thai rescue workers will drill a narrow shaft into a cave where 12 schoolboys and their soccer coach are believed to be trapped by flood waters, Thailand's interior minister said on Wednesday, the fourth day of a search that has been hampered by heavy rain.

The boys, aged between 11 and 16, and their 25-year-old assistant coach, went missing on Saturday after soccer practice when they set out to explore the Tham Luang cave complex, even though it is known to be prone to flooding in the rainy season.

Thai volunteers and military teams, including 45 Navy SEAL unit members, have been deployed at the flooded cave complex, which runs 10 km (6 miles) under a mountain in the northern province of Chiang Rai.

"Tomorrow we can drill into the mountain but we won't drill too deep. Just enough to allow people through," interior minister Anupong Paochinda told reporters in Bangkok.

"We are trying every way to find the children," he said.

While distraught relatives and friends gathered at the mouth of the cave, rescue workers pumped out water, but persistent heavy rain has slowed their progress.

"Water is the biggest challenge. There is a lot of debris and sand that gets stuck while pumping," Army officer Sergeant Kresada Wanaphum told Reuters.

"We have to switch out units because there is not enough air in there," he said before heading back down the cave.

According to messages the boys exchanged before setting off, they had taken flashlights and some food.

Apart from some footprints and marks left by their muddy hands near the cave entrance, nothing has been seen or heard of them since Saturday evening, and the race to find them has dominated Thai news.

"I'm confident all are still alive," Prime Minister Prayuth Chan-ocha told reporters.

'MASSIVE AMOUNTS' OF WATER

Vern Unsworth, a British cave explorer based in Chiang Rai who has joined the search, said a lot of water was seeping into the cave from two directions.

"There is a watershed inside, which is unusual. It means there is water coming in from two directions," Unsworth told Reuters. "The biggest challenge is the water. Massive amounts."

Three divers coming from Britain were expected to reach Thailand on Wednesday evening to join the search, the interior minister said.

The U.S. military has sent a search and rescue team at the request of the Thai government, Lieutenant Commander Nicole Schwegman, a spokeswoman for the U.S. Indo-Pacific Command, told Reuters.

Thailand has asked the United States for survivor detection equipment, Tourism Minister Weerasak Kowsurat said.

"We hope this equipment will allow us to locate the spots that we need to reach faster," Weerasak told reporters.

A guidebook described the Tham Luang cave as having an "impressive entrance chamber" leading to a marked path. It then described the end of the path and start of a series of chambers and boulders.

"This section of the cave has not been thoroughly explored. After a couple of hundred metres, the cave reduces in size to a mud-floored passage 2 metres wide and 3 metres high," author Martin Ellis wrote in "The Caves of Thailand Volume 2."

Nopparat Kantawong, the head coach of the team who did not attend practice on Saturday, said the boys had visited the caves several times, and he was hopeful they would stick together and stay strong.

"They won't abandon each other," Nopparat told reporters.