"We expect the South Korea-US summit to play a role as a bridge (between the US and North Korea) that will lead to the success of the North Korea-US summit as it comes three weeks before the North Korea-US summit," said Nam Gwan-pyo, a deputy director of the presidential National Security Office.

Moon has already held two telephone conversations with Trump since his summit with Kim.

His US visit came as he seeks to provide a more detailed account of his encounter with the reclusive North Korean leader that Cheong Wa Dae officials said could not be delivered soundly over the phone.

"President Moon will likely tell President Trump what to expect and what not to expect from Kim," a Cheong Wa Dae official said earlier.

His trip also follows what appeared to be a change of heart in North Korea's denuclearization commitment.

In a joint declaration reached at the latest inter-Korean summit, Moon and Kim agreed to pursue complete denuclearization of the Korean Peninsula and also to halt their countries' hostile acts against each other and instead put their bilateral relationship back on the right track, which called for a resumption of dialogue, including the Red Cross talks.

Pyongyang, however, indefinitely suspended its high-level dialogue with the South scheduled for last week.

It cited an ongoing joint military exercise of the South and the US as a reason, apparently throwing off many South Koreans who believed the old issue would no longer get in the way of the divided Koreas since Kim told Moon's top security adviser Chung Eui-yong in early March that he "understood" the need for the allies to resume their joint drills following a temporary suspension over the 2018 PyeongChang Winter Olympic Games held in South Korea.

The North has also threatened to reconsider holding its first-ever summit with the United States.  

While aboard South Korea's Air Force One en route to Washington, Chung said the Moon-Trump talks will likely focus on ways to keep the US-North Korea summit from falling apart.

"The key agenda of the South Korea-US summit is not the meeting of the two leaders itself but the exchange of ideas between the two leaders on how best to keep the dialogue (with North Korea) going forward," he told reporters.

Still, the chief of South Korea's presidential National Security Office said no changes have been made so far to the scheduled US-North Korea summit, also dismissing earlier reports that Trump may be having second thoughts about his meeting with Kim after Pyongyang said it may rethink the summit.

Such reports came one day after Moon and Trump held their latest telephone conversation, in which Trump reportedly inquired about a possible gap between what Moon has told him about the outcome of his recent summit with Kim and what the North has said over the past few days.

"There is nothing we have detected," Chung said when asked if the US president was in fact having doubts.

"We did not get any such feeling in the phone conversation between the leaders yesterday or in the process of our consultations with the White House National Security Council," he added.

What is currently keeping the upcoming summit alive is an apparent peace offer from Trump.

On Thursday, the US president said Kim will be "very, very happy" if they reached a nuclear deal at their upcoming summit.

The remarks came in response to an earlier claim by the North that the US may be seeking to denuclearize North Korea with no guarantee of benefits.

Trump said such a model will take place if he and Kim cannot reach a nuclear deal.(Yonhap)