Musk claimed he'd been given the nod for his Boring Company to dig tunnels between NY and Washington DC via Philadelphia and Baltimore - with the roughly 220-mile trip between completed in just 29 minutes.
Musk, the chief executive of electric vehicle maker Tesla Inc and rocket company SpaceX, is seeking to revolutionize transportation by sending passengers and cargo packed into pods through an intercity system of giant vacuum tubes known as the "hyperloop".
"Just received verbal govt approval for The Boring Company to build an underground NY-Phil-Balt-DC Hyperloop", the billionaire tweeted this morning, boasting that the 200-mile trip would take just 29 minutes.
Musk's Boring Company recently announced that it was working on its first tunnel, which is thought to run under the SpaceX headquarters and to a vehicle park opposite.
Its website outlines a transport system through which drivers could gain access to a network of tunnels that span the US.
The White House's Office of American Innovation, on the other hand, has confirmed that it has held "promising conversations" with Musk, but denied it had given approval, according to the Washington Post.
The company appeared to walk back Musk's declaration of "verbal approval" in a separate statement issued later Thursday, saying instead that the company has received "verbal support" from the government. The Boring Company did not immediately respond to a request for comment.More news: Samsung Galaxy J7 (2017) Poses for the Camera Again
The company added that it expects to cinch all the "formal approvals necessary to break ground" by the end of the year. That is to say, Musk originally wanted to create his tunneling business to accommodate cars.
Running from "city centre to city centre", the tunnel would have "up to a dozen" more entry and exit points along the route, Musk said in a series of tweets about the project.
Musk said in a later tweet that supporters should lobby government officials.
Musk has never hidden his lofty ambitions for Hyperloop, the magnetic levitation transport system that uses maglev carriages which "float" above tracks and move through vacuum tubes with reduced friction and can therefore travel at much higher speeds. After that, he's like to build one hyperloop system on the East Coast and one from Los Angeles to San Francisco.
There were plenty of naysayers when he announced plans to build a reusable rocket - but SpaceX stunned the world in 2015 when it became the first to successfully guide a rocket to a safe landing after launch.
And earlier this year, Musk to his plans for reusing rockets a step further when he sent one back into space.