US President Donald Trump says the US has agreed to a "massive" trade deal with Japan, one of the country's largest trading partners.

The plan would result in Japan investing $550 billion into the US and paying a 15 per cent reciprocal tariff, Mr Trump said in a post on social media.

He added that Japan would open its economy to US goods, including cars, trucks, rice and certain agricultural products.

Japan's Prime Minister Shigeru Ishiba welcomed the deal but said he needed to study the details, reports BBC.

"I just signed the largest trade deal in history, I think maybe the largest deal in history with Japan," Mr Trump touted at a White House event.

"This is a very exciting time for the United States of America, and especially for the fact that we will continue to always have a great relationship with the Country of Japan," he added.

Speaking to reporters, Japan's Prime Minister Shigeru Ishiba said: "As for what to make of the outcome of the negotiations, I am not able to discuss it until after we carefully examine the details of the negotiations and the agreement."

In a letter sent to Japan this month, Trump threatened a 25 per cent tariff on the country's exports to the US if there wasn't a new trade deal struck before August 1.

That was one percentage point higher from the 24 per cent rate announced during his so-called Liberation Day on April 2.

Autos make up more than a quarter of all Japan's exports to the United States, reports Reuters.

Japan will also increase purchases of agricultural products such as US rice, a Trump administration official said.

Mr Ishiba said the share of US rice imports may increase under its existing framework but that the agreement would "not sacrifice Japanese agriculture."

US automakers signalled their unhappiness with the deal, raising concerns about a trade regime that could cut tariffs on auto imports from Japan to 15 per cent while leaving tariffs on imports from Canada and Mexico at 25 per cent.

"Any deal that charges a lower tariff for Japanese imports with virtually no US content than the tariff imposed on North American-built vehicles with high US content is a bad deal for US industry and US auto workers," said Matt Blunt, who heads the American Automotive Policy Council which represents General Motors, Ford and Chrysler parent Stellantis.