By Chayut Setboonsarng

CHIANG RAI, Thailand (Reuters) – 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.

Soldiers take a rest in Tham Luang caves during a search for 12 members of an under-16 soccer team and their coach, in the northern province of Chiang Rai, Thailand, June 27, 2018. REUTERS/Soe Zeya Tun

“We are trying every way to find the children,” he added.

While distraught relatives and friends gathered at the mouth of the cave, rescue workers pumped water out, but the 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 added, 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 cycles.

“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.

Family members pass their time as they wait for their children near Tham Luang caves during a search for 12 members of an under-16 soccer team and their coach, in the northern province of Chiang Rai, Thailand, June 27, 2018. REUTERS/Soe Zeya Tun

“The biggest challenge is the water. Massive amounts.”

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

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

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

A guide book described the Tham Luang cave as having an “impressive entrance chamber” leading to a marked path. It then describes the end of the path and the start of a series of chambers and boulders.

“This section of the cave has not been thoroughly explored. After a couple of hundred meters the cave reduces in size to a mud floored passage 2 meters wide and 3 meters 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 was hopeful that the boys would stick together and stay strong.

“They won’t abandon each other,” Nopparat told reporters.

(Additional reporting by Amy Sawitta Lefevre, Aukkarapon Niyomyat, Panarat Thepgumpanat, Patpicha Tanakasempipat and Pracha Hariraksapitak; Writing by Amy Sawitta Lefevre; Editing by Michael Perry & Simon Cameron-Moore)