The US does not seek "regime change" in Iran despite mounting Middle East tensions, President Donald Trump said Monday after summit talks with Japanese Prime Minister Shinzo Abe, in dovish comments also praising North Korea's leader Kim Jong-un as a "very smart guy."

Iran "has a chance to be a great country, with the same leadership. We're not looking for regime change, I want to make that clear. We're looking for no nuclear weapons," said the president.

"I really believe that Iran would like to make a deal. I think that's very smart of them and I think there's a possibility for that to happen also."

He had earlier opened the door to negotiations with Tehran, saying: "if they'd like to talk, we'd like to talk also."

Washington has decided to deploy 1,500 additional troops to the Middle East amid growing friction with Iran after Trump pulled out of a landmark nuclear deal and later re-instated tough sanctions.

Trump also appeared to give backing to his host Abe to mediate, amid reports the Japanese prime minister is considering a trip to Tehran to negotiate.

"I know for a fact that the prime minister (Abe) is very close with the leadership of Iran... nobody wants to see terrible things happen, especially me," Trump said before the summit.

Trump was in Japan as the first foreign leader to visit the country's newly enthroned Emperor Naruhito, which he described as a "great honor."

In the morning, Trump, dressed in a dark suit and red tie, reviewed the Japanese honor guard and greeted dozens of Japanese and visiting US officials as a military band played.

Naruhito, wearing a light blue tie, and his wife Empress Masako, who was in a white hat and jacket, accompanied Trump and his wife Melania, who wore a summery white dress and tall red high heels.

Monday marked the start of the official program for the two leaders after a fun-filled weekend of sumo, golf and meals out. Trump said Sunday he was having a "great time" with his friend and close ally Abe.