WASHINGTON (REUTERS) - Expectations grew on Thursday (May 3) that North Korea would soon release three Americans held as prisoners as a gesture ahead of an unprecedented US-North Korea summit due to take place in coming weeks.

Rudy Giuliani, who joined President Donald Trump's legal team last month, told Fox News Channel that Pyongyang would release the three prisoners as early as Thursday.

It was not immediately clear whether Giuliani, a former mayor of New York City, had direct knowledge of negotiations around the issue.

Trump administration officials have pressed for the Americans' release as a show of goodwill by North Korea before the meeting between Trump and North Korean leader Kim Jong Un, which is expected to take place in late May or early June.

White House spokeswoman Sarah Sanders said she could not confirm details about the release, but she said Washington was "cautiously optimistic" about its ongoing talks with Pyongyang.

"Certainly, that would be an incredible step and certainly a sign of goodwill moving into the summit, moving into these discussions with North Korea. I can't comment any further at this point," Sanders said in an interview with Fox News.

The three are Korean-American missionary Kim Dong Chul; Kim Sang-duk, who spent a month teaching at the foreign-funded Pyongyang University of Science and Technology (PUST) before he was arrested in 2017, and Kim Hak Song, who also taught at PUST.

Trump did not mention Korea or the prisoners when he spoke at a prayer event at the White House on Thursday.

"Our country is doing very well. You'll see some very good announcements very shortly," he said.

Late on Wednesday, Trump said on Twitter, "As everybody is aware, the past Administration has long been asking for three hostages to be released from a North Korean Labor camp, but to no avail. Stay tuned!"

The US government is looking into reports that the Americans had recently been relocated from a labour camp to a hotel near Pyongyang, a US official said on Wednesday.

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.

DENUCLEARISATION

In preparing for the summit with Kim, Trump's administration has said it wants North Korea's "complete, verifiable, and irreversible denuclearisation" but has offered few details of the strategy it will employ at the talks.

Kim told the Chinese government's top diplomat, State Councillor Wang Yi, on Thursday that he is committed to denuclearisation, China's foreign ministry said, as diplomatic efforts to bring lasting peace to the Korean peninsula gather pace.

"Kim Jong Un said achieving the denuclearisation of the peninsula is the firm position of the North Korean side," China's foreign ministry said after Wang met Kim in Pyongyang.

North Korea announced on Sunday that it would close its main nuclear test site next month but some US officials are sceptical that Pyongyang will give up its nuclear arsenal.

Trump has promised to ensure that North Korea does not get the capability to build a nuclear bomb that can hit the mainland United States.

US Vice-President Mike Pence is postponing his plans to travel to Brazil this month in order to ensure foreign policy resources are focused on Trump's talks with Kim, Pence's spokeswoman said.

The exact date and location has yet to be fixed for the meeting between Trump and Kim, who lobbed personal insults and lambasted each other last year over North Korea's nuclear arms ambitions.

The change in tone has been helped by last week's meeting between Kim and South Korean President Moon Jae-in, at which the two pledged to work for "complete denuclearisation" of the Korean peninsula. Trump said he would maintain pressure on Pyongyang through sanctions ahead of his own meeting with Kim.

Even as Washington presses for the release of the three American prisoners, the parents of a US college student who died last year soon after being release from captivity in North Korea have sued Pyongyang over their son's death, saying Otto Warmbier was "brutally tortured and murdered".