US President Donald Trump said on Monday that he will start charging Japan a 25 percent tariff next month, if the country does not reach a trade deal with the United States. He also hit South Korea with the same rate.

He posted the new figure in a letter on social media. White House Press Secretary Karoline Leavitt said about 12 other trading partners will also get letters.

Trump later signed an executive order extending the original July 9 deadline to August 1.

Japan had been facing a 24 percent tariff. His letter said the 25 percent figure is "far less" than what is needed to eliminate the "Trade Deficit disparity" with the nation.

He warned that if Japan hikes tariffs, the US will match that amount, and tack it onto the new 25 percent rate.

Trump has been accusing Tokyo of carrying out "unfair" trade practices, specifically in the agricultural and auto sectors.

The letter noted the relationship with Japan has been "far from Reciprocal." Trump added that the tariff rate may be modified depending on the "relationship" with the country.

A senior Japanese official said Prime Minister Ishiba Shigeru's administration interprets the letter as effectively extending the deadline.

The official added that Tokyo believes there is still room for negotiation, and will continue holding discussions with the US to secure a deal.