Washington DC - President Donald Trump said Sunday the US military was planning to construct a large complex beneath the new ballroom he is building at the White House.

"The military is building a massive complex under the ballroom, and that's under construction, and we're doing very well, so we're ahead of schedule," Trump told reporters aboard Air Force One.

"It's part of it, the ballroom essentially becomes a shed for what's being built under," Trump said, without elaborating on the unprecedented arrangement.

He said information about the plan had come out recently "because of a stupid lawsuit."

Last October, the former real estate developer had an entire wing of the White House bulldozed, in order to build a vast ballroom to host receptions and state dinners.

Trump speaks frequently and in great detail about the construction work, which has thus far been undertaken without the usual byzantine vetting procedures for changes to Washington's built landscape.

"We are using onyx and stones that are incredible," he recently told a press conference dedicated in part to the war in the Middle East.

The ballroom project – one of the most ambitious undertakings at the White House in over a century – has continued to grow in scope, with its privately-funded budget doubling from $200 to $400 million.

Eager to leave his mark on the US capital, Trump has also renamed an iconic performance venue as the "Trump-Kennedy Center," and plans to build a grand arch in Washington inspired by the Arc de Triomphe in Paris.