President Trump on Wednesday expressed optimism about an upcoming summit with North Korea, but said if he doesn't think it will be successful, the summit won't happen, and if the summit isn't successful, he will "leave." Mr. Trump made the comments — along with remarks on the Russia investigation, Russia itself and trade — in a news conference with Japanese Prime Minister Shinzo Abe at Mr. Trump's Mar-a-Lago club in Florida.

Mr. Trump confirmed on Twitter early Wednesday that CIA Director Mike Pompeo went to North Korea and met with its leader Kim Jong Un earlier this month. Mr. Trump claimed that meeting went "very smoothly." On Wednesday night, Mr. Trump also spoke more about how he's approaching the planned summit.

"I hope to have a very successful meeting," Mr. Trump said. "If we don't think it's going to be successful ... we won't have it. We won't have it. If I think that it's a meeting that is not going to be fruitful, we're not going to go. If the meeting when I'm there is not fruitful, I will respectfully leave the meeting, and will continue what we're doing or whatever it is that will continue, but something will happen. So I like always remaining flexible, and will remain flexible here."

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The president has said the summit with North Korea will likely take place in early June, if not sooner. Mr. Trump said the U.S. is "negotiating" to free three Americans held prisoner in North Korea, although he did not directly answer a reporter's question as to whether he will make freeing those prisoners a condition for talks between the U.S. and North Korea.

"The fact is that they do have three prisoners," Mr. Trump said. "We have been talking about them. We are negotiating now. We are doing our very best. As you know, they've been there a very long time and it's harsh treatment."

Mr. Trump also kept his options open on the possibility of further Russia sanctions, after U.S. Ambassador to the United Nations Nikki Haley said new sanctions would be imposed Monday, and top White House officials said she got ahead of the curve. Haley responded by saying, "I don't get confused."

"We'll do sanctions as soon as they very much deserve it," Mr. Trump said of Russia on Wednesday.

Mr. Trump and Prime Minister Abe have been meeting at the president's club, Mar-a-Lago, over the last two days, discussing how to address North Korea's nuclear program, trade between the U.S. and Japan, and the Indo-Pacific region more generally.

There is still no date or location set for a summit between the U.S. and North Korea, although Mr. Trump said five locations are under consideration. Asked Tuesday if any of them are in the U.S., he responded "no."

"I will be meeting with Kim Jong Un in the coming weeks to discuss the denuclearization of the Korean peninsula," the president said Wednesday.

Mr. Trump said he hopes they live to see the day of a prosperous, peaceful Korean peninsula, and hopes their summit will succeed to result in complete, verifiable and irreversible denuclearization.

"It would be a tremendous thing for North Korea, and it would be a tremendous thing for the world," Mr. Trump said.

Abe, through a translator, noted that North Korea has made promises before and not kept them. Just because North Korea is willing to talk doesn't mean maximum pressure should be lessened, the Japanese leader noted.

Mr. Trump also addressed the special counsel's Russia probe. Asked if he has decided whether political fallout would be too great to fire Special Counsel Robert Mueller or Deputy Attorney General Rod Rosenstein, Mr. Trump didn't directly answer the question. He called the Russia probe a "hoax" and said his legal team has been very transparent.

"As far as the two gentlemen you told me about, they've been saying I'm going to get rid of them for the last three months, four months, five months, and they're still here," Mr. Trump said of Mueller and Rosenstein. He called the Russia investigation a "hoax" made up by Democrats, and claimed that his leal team, at his instruction, has been incredibly transparent in handing over more than a million documents. Mr. Trump said he has not claimed any presidential power or privilege in the probe. He also denied any suggestion that he's too easy on Russia.

"There's been nobody tougher on Russia than President Donald Trump," Mr. Trump said.

The president also criticized the United States' trade relationship with Japan — standing feet from the Japanese prime minister — claiming the U.S. has a "massive" trade deficit with that nation.

Mr. Trump began the news conference by praising the life of the late Barbara Bush, and Abe began his remarks the same way.

Meanwhile, Pompeo is undergoing the confirmation process to become secretary of state, and while his confirmation seems likely, it's not a lock. No Democrats have said they'll support him, and if Sen. Rand Paul, R-Kentucky, persists in opposing him, Pompeo will likely need a Democrat to vote in favor of confirming him. A handful of Democrats and Republicans say they're undecided. When Mr. Trump was asked about Pompeo's confirmation prospects, he expressed confidence that Paul would change his mind about Pompeo.

"I will say this about Rand Paul — he's never let me down," the president told reporters during a working lunch with Abe Wednesday. "Rand Paul is a very special guy as far as I'm concerned, he's never let me down and I don't think he'll let us down again."