"Well, it was a very nice letter. It was actually a 'happy birthday' letter, if you want to know the truth," said the president, who turned 73 on June 14. "It was my birthday. He sent me a beautiful letter -- happy birthday -- which was nice, very nice."

Asked if he sent a letter in response, Trump said, "I sent him a 'thank you' letter. Yeah, I sent him a note."

Neither side has revealed the contents of the missives, but the exchange has raised hopes the two will resume negotiations on dismantling North Korea's nuclear weapons program.

The talks have been at a standstill since the second Trump-Kim summit in February ended without a deal due to gaps over the scope of North Korea's denuclearization and sanctions relief from the US.

The North's state media said Sunday that Kim had received a letter "of excellent content" from Trump.

"Appreciating the political judging faculty and extraordinary courage of President Trump, Kim Jong Un said that he would seriously contemplate the interesting content," the Korean Central News Agency said. (Yonhap)