Japanese Prime Minister Sanae Takaichi sailed smoothly past a potential clash with Donald Trump over Iran on Thursday, though the US president whipped up a bit of a storm about Pearl Harbor. 

Days after lashing out at US allies including Japan for failing to heed his calls to help reopen the Strait of Hormuz, Trump praised Tokyo's efforts related to the Middle East war in vague terms.

"I believe that, based on statements (which) were given to us yesterday, the day before yesterday, having to do with Japan, they are really stepping up to the plate," Trump told reporters as he hosted Takaichi in the Oval Office.

After a long pause Trump then added "unlike NATO," repeating his criticism of the US-led military alliance with mainly European countries.

Trump gave few details about what help Japan might provide in securing the crucial waterway, through which one-fifth of the world's oil normally passes.

But his tone towards Takaichi was far more friendly than the tongue-lashings he has given to allies, including British Prime Minister Keir Starmer, on the issue in recent days.

"I'm very proud of you. We've become friends," Trump said of Japan's first female premier, whom he backed ahead of a landslide election victory in February.

The 64-year-old conservative was meanwhile seen giving Trump, 79, a big hug as she arrived earlier at the White House, according to footage released by one of Trump's aides.

Sending its Self-Defense Forces abroad is politically sensitive in officially pacifist Japan, as many voters support the US-imposed 1947 constitution that renounces war. 

But just over an hour before the Trump meeting, Japan and five other allies including Britain and France said they were ready "to contribute to appropriate efforts to ensure safe passage through the Strait of Hormuz."

Trump said it would be "appropriate" for Japan and other allies to contribute, noting that Japan gets around 90 percent of its oil through the strait.

World War II

But diplomacy is rarely simple with Trump, as he showed again when asked by a Japanese reporter why allies were not included in talks about the Iran strikes before they happened.

"Who knows better about surprise than Japan? Why didn't you tell me about Pearl Harbor?" Trump responded.

Takaichi sat back a little in her chair and her eyes widened as Trump mentioned Tokyo's December 7, 1941 attack on the US Pacific fleet in Hawaii, which prompted the United States to enter World War II.

But the Japanese premier appeared keen to make the most of her time with the US president, which will also include a dinner.

She was twice seen ostentatiously looking at her watch as the press conference in the Oval Office neared its end.

But the meeting appeared to highlight once again Takaichi's ability to charm Trump, following a similarly friendly meeting in Tokyo in October during which she said she would nominate him for the Nobel Peace Prize.

After the meeting, the two countries announced a $40 billion project to build nuclear reactors in Tennessee and Alabama, and a $33 billion investment in natural gas power generation facilities in Pennsylvania and Texas.

The deal came after Tokyo agreed last year to invest $550 billion in the United States through 2029 in return for lowering threatened tariffs by Washington to 15 percent from 25 percent.

Another reason Tokyo can ill afford to annoy Trump is that the United States has for decades been the guarantor of Japan's security, with 60,000 troops on Japanese soil.

The US security umbrella is particularly relevant in the face of an increasingly assertive China.