Donald Trump admits 'delay' to landmark meeting with North Korea's Kim Jong Un

US President Donald Trump has said his Singapore summit with North Korea's leader Kim Jong Un may be delayed.

President Trump said the meeting "may not work out for June 12."

Mr Trump raised doubts about the session planned for Singapore during talks with South Korean President Moon Jae-in, who came to Washington to urge Trump not to let a rare opportunity with reclusive North Korea get away.

"If it doesn't happen, maybe it will happen later," Trump said.

"It may not work out for June 12."

Mr Trump was responding to an abrupt change in tone from North Korea. Last week Pyongyang suggested the summit could be cancelled if the secretive state was pushed towards "unilateral nuclear abandonment."

Trump's remarks in the Oval Office were the strongest sign from him yet about the possibility of a delay or cancellation of what would be the first-ever summit between the leaders of the United States and North Korea.

It was unclear whether Trump was truly backing away from a summit that he is eager to hold or whether he was strategically coaxing North Korea to the table.

If the summit is called off or fails, it would be a major blow to what Trump supporters hope will be the biggest diplomatic achievement of his presidency.

Trump reiterated comments from last week, saying Kim would be safe and his country would be rich if he denuclearized.

But he cast doubt on the summit, saying there are certain conditions that must be met and if North Korea refuses, then the meeting will not take place.

Before seeing Trump, Moon met with Secretary of State Mike Pompeo and national security adviser John Bolton and urged them to speed preparations for the Trump-Kim summit.

"We South Korean people ... expect much from you. Please take care of us," Moon said, according to a South Korean government statement, in an apparent joke that nevertheless signalled the importance he places on the Trump-Kim summit.

A statement from the South Korean government said Moon sought to counter doubts about Pyongyang's intentions given its history of making promises and backtracking in international talks.

"This is the first time ever that 'complete denuclearisation' has been officially pronounced and the negotiation will be conducted with North Korea's top leader who wishes for security of the regime and economic progress, which makes it a different level from previous negotiations," Moon said, according to a government readout.

While remaining committed to the summit, Trump has privately has wondered whether North Korea is serious about denuclearizing, a senior U.S. official said.