American space entrepreneur Elon Musk tweeted that he visited the Tham Luang cave operation on Monday and Tuesday morning, and left behind a prototype mini-sub "in case it is useful" at the flooded cave where five members of a youth football team remained trapped.

"Just returned from Cave 3," Musk said.

"Mini-sub is ready if needed. It is made of rocket parts & named Wild Boar after kids' soccer team. Leaving here in case it may be useful in the future."

On Instagram, he published video of a flooded cave, with rescuers.

Cave Three is about two kilometres from the entrance of the cave network and is the base for rescuers.

The footballers still awaiting rescue are about another two kilometres further in, at a point very difficult to access.

There is no indication so far that Thai rescuers plan to use Musk's prototype.

Musk called it "basically a tiny, kid-size submarine using the liquid oxygen transfer tube of (a) Falcon rocket as hull."

He said it was light enough to be carried by two divers, robust, and small enough to get through narrow gaps.

The person inside need not swim or know how to use oxygen bottles.

He posted video of divers escorting the pod during testing in a Los Angeles swimming pool.

Last week, Musk said he was sending teams to Thailand from his private space exploration firm, SpaceX, and engineering firm, Boring Co. which is developing tunneling systems for transport projects.

While offering the mini-submarine as a potential savior, Musk used the opportunity to promote space exploration. He is also co-founder of the Tesla electric car company.

The Thai soccer team ventured into the Tham Luang cave complex after practice and became trapped by rising waters more than a fortnight ago.