U.S. President Donald Trump welcomed three former U.S. prisoners at a military base near Washington on Thursday, thanking North Korean leader Kim Jong Un for their release and sounding upbeat on a planned summit between the two sides. The prisoners, freed after U.S. Secretary of State Mike Pompeo traveled to the North Korean capital, landed at Joint Base Andrews outside Washington, where a giant U.S. flag was suspended between ladders of two fire trucks on the edge of the taxiway. "Frankly we didn't think it was going to happen and it did," Trump said after thanking Kim for releasing the men. "We’re starting off on a new footing. This is a wonderful thing that he released the folks early." Trump said he believed Kim wanted to bring North Korea "into the real world" and was hopeful of a major breakthrough at their planned meeting. "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." Trump and Kim engaged in a bellicose exchange of rhetoric last year over North Korea's development of nuclear missiles capable of reaching the United States in defiance of U.N. Security Council resolutions. Tensions began to ease, coinciding with the North's participation in the Winter Olympics in South Korea in February. Details of the planned summit have yet to be announced, but one U.S. official said Singapore had emerged as the most likely venue. Trump said it would be held in a few weeks' time. A White House spokeswoman said the three former prisoners would be taken to Walter Reed National Military Medical Center in nearby Maryland for further medical evaluation.