President Donald Trump said Japan is not required to protect the United States militarily and makes "a fortune" from it economically, as he fired off an impromptu broadside at a key ally.

It came as Japan's trade minister is arranging a trip to Washington during which he will reportedly demand an exemption from imminent US tariffs on steel and aluminum.

"We have a great relationship with Japan. But we have an interesting deal with Japan that we have to protect them, but they don't have to protect us," Trump said Thursday.

"And by the way, they make a fortune with us economically," he said. "I actually ask, who makes these deals?"

In response, government spokesman Yoshimasa Hayashi said Friday that Japan trusts Washington to keep its obligation to the two countries' security treaty.

Around 54,000 US military personnel are stationed in Japan, mostly in the Okinawa region east of Taiwan.

Meanwhile Tokyo's economy, trade and industry minister Yoji Muto told reporters that he hoped his US trip would "be a win-win for both Japanese and US national interests."

He said the visit was being coordinated and did not confirm the March 9-13 dates given by Japanese media or reports that he will push US officials to exempt Japan from levies.

Trump has said 25-percent steel and aluminum tariffs will be imposed on Wednesday of next week, without exceptions.

"It is important to closely study the specific content of these (tariff) measures and their impact," Muto said Friday.

He also poured cold water on Trump's announcement that Japan was among the countries looking to invest trillions of dollars in a "gigantic" natural gas pipeline in Alaska.

"This is an issue that the government and the private sector need to study, in terms of profitability and when supply will begin," Muto said.

"I hope to hear more details on this project" from US officials, he added.

Trump has also threatened to impose tariffs of around 25 percent on auto imports. Vehicles represented nearly a third of all Japan's exports to the United States last year.

Without referring to Muto's planned visit, Prime Minister Shigeru Ishiba told parliament on Wednesday that Japan's "contribution to the US economy is significant."

"So we want to appeal resolutely, with emotion and logic," to Washington on the tariffs, he said.

Muto and Japan's foreign minister will hold economic security talks with their British counterparts in Tokyo Friday, touted as a chance to promote free trade and boost business ties.