A team of US military personnel and British divers have joined the search at a flooded cave in northern Thailand for 12 children and their football coach who have been trapped for five days.

Rescuers are preparing to drill a shaft into the Tham Luang cave in Chiang Rai province.

Major Buncha Duriyapan, commander of the 37th Military District in Chiang Rai, said workers would drill down from the top of the cave complex to create an alternative entrance for rescue workers.

Rescue workers have scoured the top of the mountain looking for alternative entrances to the cave.

Authorities say they are optimistic the boys are still alive.

Heavy rain has hampered the search as floodwaters seeped into a second chamber of the cave forcing some of the rescue divers to turn back, according to officials.

The children, aged between 11 and 16, went into the cave on 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 mobilised for the around-the-clock rescue in a remote border mountainous 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 an embassy spokeswoman.

"Operators are trained in personnel recovery tactics and techniques and procedures," Jillian Bonnardeaux said.

"Essentially what they're looking at is assessing with the Thai authorities the potential courses of action and complementing the efforts under way," she added.

Thai soldiers relay electric cable deep into the Tham Luang cave
Thai soldiers relay electric cable deep into the Tham Luang cave 

Three British cave diving experts arrived at the scene late yesterday and entered the cave in full kit before emerging about an hour later.

British cave-divers, Richard William Stanton (L), Robert Charles Harper (C) and John Volanthen (R) have joined the rescue 

"We've got a job to do," diver John Volanthen said as he went into the tunnel.

Exhausted relatives have been camped out near the cave's entrance for days desperately awaiting news about the missing team and their 25-year-old coach.

"I'm sad. I want (my son) to be safe, we've heard nothing from officials yet," Mr Thinnakorn, the father of a 12-year-old in the cave said.

Water levels rose overnight even as several high-pressure water pumps were installed inside the cave to drain the pools.

The flooding kept rescuers out of the tunnels.

"The divers are now ready to go in as soon as the water drops to a suitable level," Thai NavySEAL said on their Facebook page today.

Border guards with police dogs scoured the site for new openings into the cave, which is several kilometres long.

Families brought clothing belonging to the kids to help the sniffer dogs find the team.

Search teams found three new holes this week, but only one of the chimneys was accessible.

The Tham Luang cave is a popular draw for local visitors during the dry season, though a sign at the entrance warns tourists not to enter during the wet season from July to November.

Officials said the football team and their coach Ekkapol Janthawong have been in the cave before and know the site well.

Photos on Ekkapol's Facebook page showed him with some young footballers in the cave in 2016.

The harrowing search has captivated Thailand, where local media is blanketed with coverage of the boys.

The complex cave is infamous for being a tough site for skilled divers because of its complicated network of tunnels and pools.