Washington DC - Donald Trump made an unexpected appearance Tuesday on a White House rooftop and joked about installing nuclear missiles at a time of escalating tensions with Russia.

The US president, flanked by bodyguards and under the protection of snipers, strolled for 20 minutes on the flat roof above the press room as he looked across to the site of his proposed new ballroom.

Asked why he was on the roof, he replied he was just "taking a little walk."

Quizzed about what he intended to build, he joked "nuclear missiles," before appearing to mimic the launch of a weapon with his arm.

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The US and Russia have engaged in heated rhetoric for several days.

Trump last week ordered the deployment of two nuclear submarines near Russia in response to remarks he deemed provocative by former Russian president Dmitri Medvedev.

With his planned ballroom and other projects, Trump is determined to leave an imprint on the White House, drawing inspiration from the opulent style of his private properties.

He has paved the lawn of the famous Rose Garden and adorned the Oval Office with lavish golden ornamentation.

The 79-year-old leader recently announced plans for the ballroom, in what is shaping up to be the most significant project at 1600 Pennsylvania Avenue in over a century.

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He has vowed that he – with perhaps some help from private donors – would pay for all the work, estimated at $200 million.

"It's just another way to spend my money for the country," he said Tuesday.