With little public sign of progress toward Trump's aim of Korean denuclearization, the White House said the letter followed up on the pair's June 12 summit.

"A letter to President Trump from Chairman Kim was received on August 1," said White House spokeswoman Sarah Sanders.

"The ongoing correspondence between the two leaders is aimed at following up on their meeting in Singapore and advancing the commitments made in the US-DPRK joint statement."

Sanders said a reply was being issued.

Trump alluded to the letter in a late-night tweet thanking Kim for this week's return of remains of dozens of US soldiers killed during the Korean War.

"Thank you to Chairman Kim Jong Un for keeping your word & starting the process of sending home the remains of our great and beloved missing fallen! I am not at all surprised that you took this kind action," Trump said.

"Also, thank you for your nice letter -- l look forward to seeing you soon!"

The White House later rowed back Trump's suggestion of a meeting. There is "no meeting planned," Sanders told reporters.

"There is not a second meeting that is currently locked in or finalized. Certainly open to that discussion, but there isn't a meeting planned."

"We have responded to Chairman Kim's letter, the president has. That letter will be delivered shortly," she added.