The official said the administration had hoped any announcement about the Americans' release would come from the President himself.
White House Press Secretary Sarah Sanders told reporters that the release would be "a sign of goodwill" moving into the planned summit between the U.S. and North Korea.
Rudy Giuliani, a member of President Trump's legal team but not himself an employee of the government, said Thursday morning the release would happen within the day.
South Korea's Yonhap news agency has reported that Mr Kim is in his late 50s, and said he had been involved in relief activities for children in rural parts of North Korea.
CNN, citing an unnamed source, said on Thursday the prisoners' release was imminent, adding the groundwork for the move was laid two months ago when North Korea's foreign minister traveled to Sweden and proposed the idea.
Trump took to Twitter on Tuesday, writing that "the past Administration has always been asking for three hostages to be released from a North Korean Labor camp, but to no avail". He was jailed after trying to steal a propaganda sign from a hotel in the country. And this is what Curtis LeMay, who went on to head the US Strategic Air Command, wrote: "We burned down just about every city in North Korea and South Korea both.we killed off over a million civilian Koreans and drove several millions from their homes, with the inevitable additional tragedies bound to ensue". "North Korea used this approach many times before", said Balazs Szalontai, an associate professor at Korea University.
"We got Kim Jong Un impressed enough to be releasing three prisoners today", Giuliani said on "Fox & Friends".
President Donald Trump and his allies outside the government sought to fan the heightened expectations, casting the potential development as evidence of his negotiating prowess ahead of a summit with North Korean dictator Kim Jong Un. He also agreed to halt testing of intercontinental ballistic missiles and nuclear bombs.
Sources familiar with the discussions declined to say whether Trump was looking at a complete or partial withdrawal. This was key. This meeting was the result of a series of bilateral meetings between North and South Korea's officials, which really should have given it away.
China sent troops to fight on North Korea's side in the 1950-53 Korean War, but relations have suffered in recent years over the North's reluctance to implement Chinese-style economic reforms and Kim's continued pursuit of nuclear bombs and the missiles to deliver them.
North Korea was able to test its ICBMs and a thermonuclear weapon before Kim agreed to suspend the tests.
He was arrested in October 2015, after he reportedly received a USB stick containing nuclear-linked data and other military information from a former North Korean soldier.
Trump's economic pressure on China in particular may have helped persuade Kim to move toward diplomacy, Szalontai said.
"China is willing to strengthen communication with North Korea and continue to play a positive role in seeking a political solution to the Korean peninsula issue", he said.
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