Tech entrepreneur Elon Musk on Thursday said he had received "verbal" approval to start building an ultra-high-speed underground transport system linking New York and Washington that would cut travel time between the cities to about half an hour.

He offered no details on what entity had greenlighted the project that could result in the world's longest tunnel.

Musk, the chief executive of electric car maker Tesla 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".

Officials in Washington and New York said they had not approved any project, and under federal rules Musk would need extensive environmental and building permits to mount such an ambitious project.