US President Donald Trump says the massive arch he wants to build in Washington DC will become a “top grade military complex”, able to host drones and snipers while storing ammunition.

It is one more example of how Trump is insisting that his initiatives to beautify the White House and the city are also serving a defensive purpose.

The Pentagon said it had no information about the arch – planned for an area between the Lincoln Memorial and Arlington National Cemetery – beyond the President’s statement on social media on Sunday (US time).

But Trump’s announcement came as the arch, like his other projects, faces legal challenges.

There has also been criticism it will potentially obscure views of the Arlington National Cemetery, the US’s famed resting place for war veterans.

Though the arch has received early approval from the US Commission of Fine Arts, whose members were all tapped by Trump, a group of three veterans and an architectural historian sued, saying it needs the approval of Congress.

“The President’s belated efforts to manufacture a national security justification for this project do nothing to cure its fundamental illegality,” Nicolas Sansone, one of the lawyers representing the litigants, said on Sunday (US time).

“Congress has not authorised the arch and unless and until Congress does so, the project should not be permitted to move forward.”

In a social media post, Trump said he had agreed – at the “strong request” of the military – to convert the planned 76-metre memorial arch “into a top grade Military Complex/Triumphal Arch, to house, store, and have the rapid ability to use large numbers of drones, plus Snipers, on both the roof and plaza areas, and additionally have and hold large quantities of sniper ammunition in storage”.

The arch is proposed for a circle adjacent to the Memorial Bridge, which is a heavy traffic area as one of a handful of connecting bridges between the US capital and northern Virginia.

The project needs to be approved by US Congress. Photo: AAP

Trump first proposed building a monumental arch last year to mark 250 years of US independence, saying it would be akin to other famed arches like the Arc de Triomphe built in Paris after World War II, but “the biggest one of all”.

Trump’s Interior Secretary, Doug Burgum, announced on social media earlier in September that construction would soon begin on the arch.

“After a very long wait (over 125 years!), we are preparing to start, over the next two-week period, the excavation work necessary for the Great Triumphal Arch,” he wrote.

“This will be one of the Great Pieces of American Architecture, honouring the history and significance of Arlington Cemetery and befitting the most powerful Capital in the World.”

The project is one of several that Trump is pursuing to leave his lasting imprint on Washington.

Among the others are the White House ballroom, renaming and renovating the Kennedy Centre, refurbishing the Lincoln Memorial Reflecting Pool and rebuilding a golf course in East Potomac Park, which could significantly reduce the public’s access to running and biking paths.

The arch has not yet received final approval from the National Capital Planning Commission, which greenlit the site and preliminary plans at its July meeting.

-with AAP