Tech billionaire Elon Musk says he has been building a "tiny, kid-size submarine" with "feedback from Thailand" in an effort to rescue 12 boys and their soccer coach trapped in a cave.

Mr Musk said it would use a SpaceX Falcon rocket's "liquid oxygen transfer tube as [the] hull".

"Light enough to be carried by two divers, small enough to get through narrow gaps," the Tesla and SpaceX entrepreneur posted on Twitter.

He said the design had four "handles/hitch points" on both the front and back of the submarine.

"Two air tank connections on front and two on rear, allowing one to four tanks simultaneously connected, all recessed for impact protection [with] secondary cap seal if leak develops," Mr Musk said.

It also has "segmented compartments to place rocks or dive weights [and] adjust buoyancy".

A 3D graphic of Tham Luang cave system in Thailand shows where the team is trapped.

Mr Musk said he was constructing the mini-submarine in Los Angeles with his SpaceX team, and expected it to be completed by about 12:30pm AEST.

He said it would then take 17 hours to fly it to Thailand.

He said he was also making an "inflatable tube with airlocks", which was "less likely to work, given tricky contours".

Earlier, Mr Musk said he was sending engineers from his Boring Company — which digs tunnels for advanced transport systems — and SpaceX to Thailand "to see if they can be helpful".

They were expected to reach the cave today.

The boys have been trapped in the cave for two weeks. ( AP: Royal Thai Navy Facebook page )

The Thai Government said Mr Musk's team could help the rescue operation with location tracking, water pumping or battery power.

The boys, 11 to 16, and their 25-year-old coach went exploring in the cave after a soccer game two weeks ago, when monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.

With oxygen levels dropping and rain forecast, time has been running out on a plan to teach the boys — not strong swimmers — to make a dive through narrow, waterlogged passageways that would challenge experienced cavers.

Rescue teams have been making preparations to save the boys, with the leader of the rescue effort saying the best window for a rescue could come in the next three or four days.

Time has been running out on a plan to teach the boys. ( AP: Royal Thai Navy )

ABC/Reuters