US President Donald Trump said Wednesday that although hes looking ahead optimistically to a historic summit with North Koreas Kim Jong Un, he warned he was not afraid to walk away if the meeting appears unlikely to be "fruitful." The odds of the Trump-Kim talks taking place were boosted by the shock news of secret Easter weekend talks between CIA chief Mike Pompeo and the reclusive thirty-something strongman -- the most significant US-North Korea contacts in almost two decades. Pompeo raised the plight of three US citizens held in North Korea during his meeting with Kim Jong Un, a senior White House official, who was not authorized to publicly discuss the sensitive discussions, told The Associated Press on condition of anonymity. At a press conference Wednesday with Japanese Prime Minister Shinzo Abe, Trump said the US and North Korea are negotiating for their release. He said: "We are doing our very best." The official said Trump is hopeful they will be released soon. Trump had said he thinks "theres a good chance of doing it" around his potential talks with Kim. Trump earlier tweeted that "details of Summit" between him and Kim were "being worked out now," with five possible locations being considered. Ever disposed to set out the options in the starkest terms possible, Trump predicted a "very successful" May or June summit with Kim. Trump also pledged to push Kim to free Japanese abductees held in North Korea. But in the same breath, he warned: "If I think that its a meeting that is not going to be fruitful, were not going to go. If the meeting, when Im there, is not fruitful, I will respectfully leave the meeting." "I like always remaining flexible, and well remain flexible here," Trump added.