Trump, accompanied by his wife Melania, became the first world leader to meet with newly enthroned Naruhito and Empress Masako on Monday -- an honour that Japanese Prime Minister Shinzo Abe said underlined the two countries' deep economic and strategic ties.

There has been little substance in the visit, which started Saturday and included an Abe-Trump golf game, as well as the presentation by Trump of a huge trophy at a sumo tournament.

Tricky negotiations over Washington's insistence that Japan open markets to more US products or face steep auto tariffs have been all but set aside until after elections here in July.

But more than anything, Trump's presence in Japan and his hosts' energetic efforts to please him were meant to signal that, at a time of ever greater Chinese influence and North Korean nuclear tensions, the United States remains a major Asian power.

Trump was rounding off his trip Tuesday by demonstrating the rawer side of that power with a stop at the Yokosuka US Naval Base.

He was to inspect the Japanese helicopter carrier Kaga, then address service members on the US warship Wasp.

Coinciding with the US holiday of Memorial Day, which commemorates the war dead, the naval visit will give Trump a chance to show off the American flag and underscore how one-time World War II foe Japan is part of the bedrock of the US Pacific presence.

The military alliance also has a strong commercial aspect, as Trump was eager to point out Monday when he confirmed Japan's plan to buy 105 F-35 stealth warplanes.

"This purchase would give Japan the largest F-35 fleet of any US ally," Trump said.

After leaving the USS Wasp, Trump will then depart for Washington, with a refuelling stop at an air force base in Alaska.

- Feasting and farewell -

Emperor Naruhito took over the Chrysanthemum Throne only three weeks ago, after his father stepped down in the first abdication in two centuries.

He hosted the Trumps at his palace Monday morning and then again in the evening for a banquet featuring six courses, including Trump's favourite -- beef -- and a dessert described as Glace Mont Fuji.

The emperor and Trump both made toasts praising their countries' friendship. The US president even sprinkled a few Japanese words into his address, referencing ancient Japanese poetry.

Trump and the first lady said goodbye to the Japanese royal couple on Tuesday before leaving Tokyo. The White House only described this as a "farewell call", and there were no immediate details on how it went.

Although the whole Japan trip was designed to be a feelgood display of US-Japanese friendship, there was an awkward moment Monday when Trump flatly contradicted Abe and some of his own advisors.

Trump insisted that he does not consider recent North Korean short-range missile tests to have violated UN resolutions, or even to pose a particular threat.

"My people think it could have been a violation... I view it as a man who perhaps wants to get attention," Trump said of North Korean leader Kim Jong Un, whom he once more praised, calling him "very smart".