US President Donald Trump said his administration will impose 15 per cent "reciprocal" tariffs on Japanese products, 10 percentage points lower than what was previously announced, as he touted a "massive" trade deal with the Asian ally.

Trump made the announcement in a social media post as South Korea and other countries are striving to reach trade deals with the United States to avoid or reduce the steep tariffs that his administration plans to start implementing on Aug. 1 unless trade deals are agreed on.

"Perhaps most importantly, Japan will open their Country to Trade including Cars and Trucks, Rice and certain other Agricultural Products, and other things. Japan will pay Reciprocal Tariffs to the United States of 15%," he wrote on Truth Social, reports Yonhap news agency.

In a letter to Japanese Prime Minister Shigeru Ishiba earlier this month, Trump said the U.S. will charge Japan 25 percent reciprocal tariffs, a percentage point higher than what Trump initially announced. He has also said that his government will impose 25 percent tariffs on Korea.

Trump described the deal with Japan as "perhaps the largest" deal ever made, noting that the Asian country pledged to make a US$550 billion investment in the U.S., which he said will receive 90 percent of the profits. He did not elaborate on the investment plan.

"This Deal will create Hundreds of Thousands of Jobs -- There has never been anything like it," he said.

Washington and Tokyo had struggled to break an impasse at a time when the Japanese government was thought to have been in a difficult position to show flexibility in the trade negotiations ahead of its parliamentary elections held over the weekend.

Last week, Trump said that his administration might "live by the letter" on the 25 percent tariff rate for Japan, hinting at difficulties in hashing out a deal.

In its trade talks with the U.S., Seoul hopes to secure exemptions or relief from the reciprocal tariffs as well as sector-specific duties on steel, aluminum and automobiles, which are bound to impact Korea's export-reliant economy.

Earlier in the day, Trump also announced a deal with the Philippines, saying the Southeast Asian country will pay 19 percent tariffs, a percentage point lower than what was previously announced.