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 press secretary 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."

Trump alluded to the letter in a late-night tweet thanking Kim for the return of remains of dozens of US soldiers killed during the Korean war, which arrived in Hawaii on Wednesday.

"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!"