U.S. President Donald Trump said Sunday he was"surprised" to learn that Japanese Prime Minister Shigeru Ishiba has decided to step down less than a year after taking office.

"I was surprised, because I knew him, I liked him, and he's just now stepping down," Trump told reporters as he traveled back to the Washington area from watching the U.S. Open tennis men's singles final."I found him to be a very nice man, actually. We dealt very well together."

Asked by a reporter whether he plans to visit Japan this fall along with South Korea, which will host this year's summit of the Asia-Pacific Economic Cooperation forum, Trump said,"We'll see."

A U.S. State Department spokesperson, meanwhile, told Kyodo News, regardless of Ishiba's decision,"We look forward to continuing our work with the government of Japan."

Ishiba held an urgent press conference on Sunday evening in Tokyo, saying he would resign as both prime minister and leader of the Liberal Democratic Party.

Hours before leaving for New York to attend the tennis match, Trump said he was unaware of Ishiba's intention to resign, in a sign that the president was not briefed in the morning about the Japanese leader's announcement.

"I don't know anything about that," Trump told reporters at the White House when asked about the news regarding Ishiba, who was unable to fend off pressure from members of his own party to take responsibility for the loss of the ruling coalition's House of Councillors majority in an election in July.

The result followed the coalition's failure last year to retain its majority in the more powerful House of Representatives, leaving Ishiba struggling to lead a minority government.

Ishiba's short-lived premiership is likely to prolong the political uncertainty in Japan, although U.S. officials have said Washington's alliance with Tokyo has never been stronger, calling it"the cornerstone of peace, security, and prosperity in the Indo-Pacific and across the world."

Ishiba said Sunday he had decided to avoid a "decisive split" within the party after facing persistent calls from his opponents to take responsibility for the outcome of the election, in which the LDP and its coalition partner, the Komeito party, lost control of the upper house.

The premier said it was the appropriate time to announce his departure from office as Japan's tariff negotiations with the United States had concluded.

Trump signed an executive order on Thursday formally implementing a trade deal that his administration struck with Japan in July.

Following multiple rounds of negotiations, the Trump administration finalized tariff relief long sought by Japan, including reducing the U.S. duty on automobiles from Japan to 15 percent from the current rate of 27.5 percent.

Ishiba, who took office in October, has held in-person talks twice with Trump, first in Washington in February and second in the Canadian mountain resort of Kananaskis in June on the sidelines of a Group of Seven summit.

© KYODO