Washington DC - President Donald Trump showed off a giant new helicopter landing pad at the White House on Wednesday, the latest in a series of construction projects, as he seeks to leave his mark on Washington.

While the Iran war is stalled abroad and the economy stalls at home, the former property tycoon seemed reinvigorated as he led journalists on an impromptu tour of the new landing zone on the South Lawn.

"One thing I know how to do is build!" Trump told reporters.

Trump said the granite helipad was necessary because the new versions of the Marine One helicopter coming into service would burn or damage the grass on the iconic lawn at the back of the White House.

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"So now with the helipad, we won't have any problem with the grass," Trump said.

The 80-year-old, who made his fortune in the New York real estate business, also confirmed reports that he had ordered the original work on the landing spot to be redone, saying it was originally on too much of a slope.

It would be ready in time for the arrival of Chinese President Xi Jinping on September 24, he said.

Trump then signed the back of a stone shaped like an eagle's head that will be inserted into the pad, which is a giant replica of the seal of the president of the US.

To build the helipad, workers have torn up much of the historic lawn that has played host to moments like the departure of President Richard Nixon after his resignation in 1974 and the signing of the 1993 Oslo Accords.

Critics have slammed Trump's building frenzy during his second term in the White House, including the demolition of the East Wing to build a massive ballroom.

Trump boasted about the scale of the ballroom project, which he says will include a top-secret underground bunker and a drone port on the roof.

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"This will be the greatest military complex-slash-ballroom anywhere in the world," he said.

But the ballroom has been tied up by legal action, and the Trump administration last week asked the Supreme Court to allow it to complete construction on national security grounds.

Trump has insisted that the ballroom will be privately financed, but media reports in June showed that significant taxpayer funds were going to the project, which could cost as much as $600 million.

It is just one of Trump's pet projects as he seeks to overhaul Washington by the time his second term ends in January 2029, along with a renovation of the Lincoln Memorial Reflecting Pool and other parks, and construction of a triumphal arch.