WASHINGTON (Reuters) - U.S. President Donald Trump said on Thursday he had high hopes of “doing something very meaningful” to curtail North Korea’s nuclear ambitions at a summit in Singapore next month, after Pyongyang smoothed the way for talks by freeing three American prisoners.

The date and location of the first-ever meeting of a sitting U.S. president and a North Korean leader were announced by Trump on Twitter.

“The highly anticipated meeting between Kim Jong Un and myself will take place in Singapore on June 12th. We will both try to make it a very special moment for World Peace!” Trump wrote.

He made the announcement after a U.S. government aircraft touched down at Joint Base Andrews near Washington carrying the Americans who were released by North Korea in a move to clear the way for the bilateral summit.

The ex-prisoners are Korean-American missionary Kim Dong-chul, who was sentenced in 2016 to 10 years’ hard labor; Kim Sang-duk, also known as Tony Kim, who taught for a month at a foreign-funded university before he was arrested in 2017; and Kim Hak-song, who also taught there and was detained last year.

North Korean state media said they were arrested for subversion or “hostile acts” against Pyongyang.

Trump faces a difficult task persuading Kim to abandon nuclear weapons and ballistic missile tests that heightened U.S.-North Korean tensions throughout 2017.

The two men exchanged fiery rhetoric last year over North Korea’s attempts to build a nuclear weapon that could reach the United States.

But tensions have since eased, starting around the time of the North’s participation in the Winter Olympics in South Korea in February.

Trump greeted the freed Americans in the early morning hours. He said on their arrival that he believed Kim, who has led North Korea for seven years and is believed to be in his mid-30s, wanted to bring his country “into the real world.”

“I think we have a very good chance of doing something very meaningful,” Trump said. “My proudest achievement will be - this is part of it - when we denuclearize that entire peninsula.”

Slideshow ( 3 images )

New U.S. Secretary of State Mike Pompeo has visited Pyongyang twice in recent weeks - once as head of the CIA - but there has been no sign he cleared up the central question of whether North Korea will be willing to bargain away nuclear weapons that its rulers have long seen as crucial to their survival.

Trump is embarking on the meeting with Kim after announcing on Tuesday the United States was pulling out of a 2015 accord imposing international oversight of Iran’s nuclear program.

The move raised questions over whether North Korea might now be less inclined to negotiate its own nuclear deal with Washington.

Trump and Japanese Prime Minister Shinzo Abe spoke by telephone on Wednesday. The White House said they “affirmed” the shared goal of North Korea abandoning its nuclear weapons program.

Japan worries it could be the target of any first use of nuclear weapons by Pyongyang.

White House officials, without providing specifics, said on Thursday the meeting could be scuttled if North Korea did anything unacceptable over the next month.

“We’re not under any illusions about who these people are,” National Security Council official Victoria Coates told reporters. “We know who we’re dealing with here. But we got up front our people home rather than that having been an afterthought.”

‘PHOTO OP’ WORRY

Chuck Schumer, the top Democrat in the U.S. Senate, warned Trump against going too far, too fast in Singapore. The Republican president, he said, should insist on strong, verifiable disarmament commitments from North Korea.

“I worry that this president, in his eagerness to strike a deal and get the acclaim and a photo op, will strike a quick one and a bad one, not a strong one, not a lasting one,” Schumer said.

Slideshow ( 3 images )

During Trump’s presidency, Kim has overseen a series of weapons tests to showcase his military’s progress on medium- and long-range missiles and atomic weapons that put the world on edge.

Last year, North Korea conducted more than a dozen tests that had missiles flying over the Sea of Japan, while another led experts to believe North Korea could possibly hit the mainland United States with a missile.

Trump has credited a U.S. “maximum pressure” campaign for drawing North Korea to the negotiating table and vowed to keep economic sanctions in place until Pyongyang takes concrete steps to denuclearize.

But former spy chief Kim Yong Chul, director of North Korea’s United Front Department, said in a toast to Pompeo over lunch in Pyongyang this week: “We have perfected our nuclear capability. It is our policy to concentrate all efforts into economic progress.”

Kim recently promised to suspend missile tests and shut a nuclear bomb test site.

The choice of Singapore will put the summit on friendly turf for Trump, as the island nation is a strong U.S. ally and the U.S. Navy frequently visits its port. White House spokesman Raj Shah said Singapore was chosen because it could ensure the security of both leaders and provide neutrality.

The wealthy financial and shipping hub is seen as a gateway between Asia and the West and has been called the “Switzerland of Asia,” in contrast to North Korea’s isolated economy that its leaders now want to modernize.

Nonetheless, Human Rights Watch has described Singapore as having a “stifling” political environment with severe restrictions on “basic rights.”

Singapore said in a statement late on Thursday it was pleased to host the meeting between Trump and Kim, adding: “We hope this meeting will advance prospects for peace in the Korean Peninsula.”

U.S. officials had looked at several sites other than Singapore for the historic meeting.

Trump’s own preference was for the demilitarized zone between the two Koreas, but aides argued it would look too much like Trump going to Kim’s turf.

Kim and South Korean President Moon Jae-in held a rare round of talks at their heavily fortified demilitarized zone at the end of April, pledging to pursue peace after decades of conflict.

South Korea said on Thursday: “We hope the denuclearization of the Korean peninsula as well as permanent peace on the peninsula will successfully come about through this summit.”